How long is a yeast srarter good for?

I have a 2 liter starter of wlp002 in my fridge that has been there for around 2 months and wanted to know if I decant and add more starter Wort will the yeast still be good?

well im gonna go ahead and decant and add more wort, what should i be looking for as signs that the yeast is still good?

I think yeast loss some % of vitality per week in the fridge, either 5 or 10 percent. This means after 2 months, assuming 10%, your yeast is almost 20% of what you had at the start (technically it is more, but simple math at the moment). Anyhow, your method of decanting and pitching on a fresh set of wort is the right thing to do. you should see fermentation like you would with any starter. At the most basic, a hydrometer test at the end (assuming you stirplate) should show a drop in gravity.


*** not so simple math... If I understand it right, you loss 10% vitality, which means you keep 90%, or .9 so .9^8 (yes 8th power for 8 weeks - 90% of 90% of 90%.... for 8 times) which gets closer to .43%... or 2/5 of whatever it was when you put it in the fridge. This all assumes that the yeast didn't get contaminated and such.

It might have been better to decant most of the "beer" 36-48 hours after beginning the starter and then adding some boiled (and therefore deoxygenated) and cooled water.

Basically, you want to make another starter out of it. Decant, swirl it up and add some new wort. You want to see krausen just as in the original starter.