Excellent question! The quick answer is- I don't think you have anything to worry about. Nothing truly bad, foul, or harmful happens. Silk gets used in medical devices that are actually implanted into people, and has been for years, so they've done a lot of research into what really happens to silk over a long period of time, and I've never ever seen any sort of research suggesting anything harmful happens. What does happen is that as a fiber it loses tensile strength as the proteins age- and it's really on a microscopic level- it's not something you'd notice. Moisture and enzymes break it down, so as long as you're handling it in a relatively sanitary way (use a swirly bowl), it's most likely going to be something that you can use for a very very long time (decades) without anything bad happening to you. We keep samples of each batch of stuff we make (it's an FDA regulation), and I haven't seen anything visibly go bad. I found part of batch #1 of Perfecting Powder the other day (well over 3 years old), and I wouldn't hesitate to use it- and it's been in my garage- so nothing fancy as far as storage or anything.
The initial batches of silk powder I got had expiration dates on them- this was 3 years ago. So when we built the site, we put that everything had a 2 year shelf life because that was what was on the packages of silk that we received. The newer batches don't have expiration dates on them, and US Pharmacopeia lists it as non-degradable. I know my supplier changed suppliers a few years ago- so I know there's a difference between what we started using, and what we use now. The powder we use is 100% fibroin- in case you're doing any pub med reading.
If you ever have insomnia- here's an article that explains the entire process:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2680630/
and I had a quick google and checked some MSDS's, there doesn't seem to be any obvious data about degradation of silk powder or timelines or anything (I am a scientist, lol, so a bit of a geek about this sort of thing) so I guess I'll take the FDA's word for it that it is going to stay good 
