How long will yeast last?

I opened a packet of yeast 2 days ago...has it gone bad yet?

 

Dry or liquid? If dry, it is probably ok.

 

It's dry yeast

 

I've used a packet of Montrachet for three batches of wine over the course of a year, no problems.

With liquid, I'd say a couple months. Yes, months. I've used half a vial or less on batches I knew would start well, and haven't had any problems. Sure, I've only done 11 batches of beer to date (not counting lots of wine) but I've done them all on 4 vials of WL. Optimally, you can cook about a gallon of starter and pour it into 4 bottles, put three in the freezer for future use, and then use a third or so of the vial to make a one quart starter.

 

My understanding is that freezing yeast kills it. The yeast crystalizes, damaging the cell membranes, or something of the sort. I'd recommend putting it in the fridge.

 

You can freeze yeast. You just to add a protectant to prevent damage due to ice formation. DMSO will do this but it isn't too easy to get or handle. Glycerol would be a good choice as it is safe to ingest.