I didn't know the best place to share this little bit of data, hopefully it won't get buried here:
An opened packet of dry yeast, properly preserved, will probably be good 6 months later.
6 months ago I opened up a packet of Lalvin 71B-1122 for a 1-gallon batch of wine. About a month later I used it for a melomel, but after that I just folded up the packet very tightly (pushing out as much air as possible and keeping it as sealed as possible) and put it in a ziploc bag in the fridge, also with as much air pushed out as possible.
I just pitched the rest of it (~ 1/2 tsp) directly into a 1-gallon batch of strawberry wine (with 1/2 tsp yeast nutrient), and was a bit worried because it didn't give off the yeasty aroma a fresh pack does. However, less than 24 hours later it's developing a krausen, more or less as quickly as brand new yeast. It's usually suggested you use dry yeast only a few weeks after it's opened, but it turns out at least in this case it wasn't really an issue using it 6 months later! If I were thinking, I would have rehydrated it first, but I've never actually bothered to do that after about a year of brewing.
Dry yeast is cheap so getting a new packet wouldn't have been an issue but I just thought I'd try it out anyways.