It depends also of what type of work you do and how you spend your day, but avreage once a week is good, i also agree with berealjohn. if you want to gain muscel you have to consider all the factors and than plan a good program. do a short term programm of a two week period if you dont get any results change program. when you get your result start the programm on a longterm to get to your goal
many programs are based on this basic setup. These types of programs just reduce the chances of you over training or getting injured by a significant amount. Also ive found these types of programs allow you to be more flexible with time.
Once a week is more than enough for strength/size gains
"in the howling bleeding nights, the dogs plunge into the Volga and swim desperately to gain the other bank. The nights of Stalingrad are a terror for them. Animals flee this hell; the hardest stones cannot bear it for long; only men endure."
Working out a body part once per week is a very common training method that work quite well for a lot of people. However, keep in mind that it is not the only one. There benefits related to training the same body part more frequently. Just make sure to adjust the volume accordingly if you decide to train bodyparts multiple times per week.
The only time it's bad to feel the burn is when you're peeing...