Although SSD technology keeps changing and improving, is there any measurable way to determine the estimated amount of time until an SSD kicks the bucket? I have heard of the write part dying but allowing you to copy data from an SSD, but is there a failure that would be just instant? Or something over time that a user could notice and decide to do their last-chance-backup then?
Most SSD failures I've seen or heard of are the instant, unpredictable kind. They are usually because of a problem with the firmware or controller. There is really no way to predict these. The best you can do is to make sure you update your firmware, and maybe staying with SSD manufacturers that have a good track record in regards to SSD reliability.
Write wearout can be predicted. Most SSDs have a SMART value called Media Wearout Indicator. It it SMART value E9 on my Intel SSD. Most SMART and SSD monitoring tools can read this value and give you a estimated life remaining in percent. Usually, but not always, the MWI is based on rated program/erase cycled, which makes the rated lifespan very conservative. Most SSDs can do 2x-7x their rated lifespan.