It usually takes two to three months to get through the entire initial training syllabus that I described in my last "Training Days" post. Thankfully, this is the longest training cycle you'll typically go through; that said, you can still end up in training for a pretty significant portion of your career, especially if you fly for a carrier that operates multiple types of aircraft.
Every time you switch between types of aircraft - from DC9 to A320, for example - you have to go through transition training. This is basically the same as initial training except that you don't have to do the Basic Indoctrination portion of ground school, just aircraft systems. Other than that the syllabus is pretty close, although a transition is really a little easier because you're already so familiar with the airline's general operating practices and fleet managers try to harmonize procedures between types. Depending on the number of aircraft types in an airline's fleet and special contract provisions like seat locks, it is sometimes possible to go through several cycles of transition training in a single year. A guy I know got hired onto the MD88 at Delta and only flew it for a month or two before going back to the schoolhouse for transition training on the B757/767.
Closely related to transition training is differences training. The FAA requires differences training when they ascertain that two or more models of the same type are different enough that a pilot already holding the type rating needs to get additional training on that specific model before operating it under FAR 121. The best example of this is the B757 and B767. They actually share a type certificate and their pilots hold a "B756/B767" type rating, but in reality there are some differences (195,000 lb difference in takeoff weight between the 757-200 and 767-400ER, for starters) that require additional training for pilots who fly both. A less obvious example is the B737-200 and the newer B737NG's, because the cockpit is so different. Differences training is typically much less intensive than initial or transition training.
There actually is one instance where a pilot already with an airline is required to go through initial training again: when they have not served on the same "group" of aircraft (turboprop, turbojet). Therefore, at Horizon, Dash-8 pilots going to the CRJ had to go through initial training again, and vice versa. It's basically transition training with a general subjects refresher course thrown in.
When a First Officer is promoted to Captain, they go through Upgrade training. I won't have to do this because I'm already being trained as a Captain; the company didn't want to send me through two training cycles within a few months of each other. Most airlines, however, train their First Officers as FOs only; there's no real advantage to having them all type rated and captain qualified (and most new FOs at the regionals today aren't even close to ATP minimum qualifications).
Upgrade training is, by itself, pretty straightforward: a "refresher" ground school, some CPT sessions to learn the left-seat flows, and then simulator training to learn how to fly from the left seat. The only thing that's really new for most people is learning how to taxi! Because of the way the seniority system works at most airlines, it's pretty common to upgrade into an airplane other than the one you flew as an FO. In this case, upgrade training can be combined with initial, transition, or differences training, as appropriate. For example: a CRJ FO upgrading in the DHC8 would go through initial + upgrade, a 747 FO upgrading in an A320 would go through transition + upgrade, and a Q400 FO upgrading into the Q200 would go through differences + upgrade (which would've been my case had I stayed at Horizon).
Now even if you're hired as a Captain at a one-type airline, you don't escape from training for years on end. There's always recurrent training to do. All flight crewmembers must go through recurrent ground school once a year. This tends to be dreadfully boring, as the FAA has a long laundry list of things that must be covered, which leaves little time for extra training on pertinent and timely issues that'd actually be useful on the line. On the plus side, you often find yourself in class with coworkers you haven't hung out with in a while, and that can be a good time.
And then there's recurrent simulator training/checking. The regs say that captains must take a proficiency check every six months and FOs once every twelve months, but that recurrent flight training may be substituted for every other pro check ("training-in-lieu"). I've discussed this before, but the pro check is actually easier than most training in lieu sessions, because a pro check is a fairly structured event in which everyone knows what to expect, while the instructor can (and will) throw pretty tough, complex situations at you in a TIL session. I actually think it's easier on Captains because they fly the sim every six months and therefore have a pretty good memory of what it flies like...not always quite like the airplane! Maybe I'll think differently once I have to visit the torture box every six months, though.
AQP programs are considerably different where recurrent training is concerned. The time intervals are different and are unique to each airline's program, although I understand that nine months is a common interval. The recurrent sim training is usually done right after recurrent ground school, which isn't always true of traditional programs. Also, AQP doesn't have training one time and checking the next; rather, pilots are trained and checked within the same training cycle. Usually there are at least two sim sessions, including one maneuvers validation and one LOFT (Line Oriented Flight Training) scenario. These are rough equivalents of the pro check and TIL, and doing both in the same cycle is why AQP programs get away with longer intervals between training cycles.
Finally, each pilot is required to be line checked once a year. This involves a line check airman sitting on your jumpseat during one or two legs of normal revenue flying and silently taking notes of what you're doing wrong. You have to do something pretty blatantly unsafe to actually bust a company line check, but at many airlines even small screwups count towards a running point total. If you reach a certain number of points over a given time period, you get hauled in for retraining or even a review board.
I just calculated how much time I've spent in training in less than four years at two airlines. My initial at Horizon took about ten weeks including IOE, I sat through three recurrent ground schools, and I had one recurrent pro-checks and two TILs. Initial here at NewCo will be about six weeks (not including the self study period) plus about a week of IOE. So that'll bring me up to about 20 weeks of training in under 4 years. That's actually not bad, I'd bet there are people who have done that in a single year. Like I said, it's a major part of an airline pilot's life - and that's a major reason the airlines are so safe.