Juan,
Not very scientific, but I have found that keeping glycin in the freezer, at least double sealed, (jar in heavy sealed plastic freezer bag) significantly slows the steady progression from light tan to "not worth a s#!t" dark brown. I also found that displacing the atmosphere with nitrogen helps both freezer storage and room temp storage.

Since I typically buy in 100g lots from PF, I think the longest that I have stored & used a supply has been about 18 months. Always worked fine.

Cheers,
Geary