I am considering plating up some yeast/isolating cultures and starting my own small yeast bank, I know that If I were to keep these at -80C in a lab freezer they would keep for years, but how long should I expect them to stay viable in a domestic fridge or freezer?
There are really too many factors to give a solid time frame when all the cells will have died.
I've had year old trub I've been able to make starters from.
If I had to pick a safe age I would reslant a precious strain ever 3-4 months.
A slant/plate made directly from a starter will likely stay viable longer than that picked up from trub. I'd estimate under decent conditions it should be good for up to about 6 months.... longer if under optimum conditions. But like Evil Zymurgist said, there are many, many factors at play here.