If I take a yeast slurry from a batch of beer, wash it and store it in a sanitized container - how long can I leave this yeast slurry in the fridge before repitching? I'm thinking of leaving it in the order of months.
The short answer is that you can leave it for 2-4 weeks in the fridge and pitch directly. Longer than that, and it's best to make a starter from a small amount of the slurry to avoid a sluggish start and yeast bite from many dead yeast cells.
I have been reusing my yeast for several years now. I don't do it correctly, but it has worked out for me. I would like to share what I actually do, and you can balance that against all the really great information regarding the proper care and feeding of yeast.
The only limiting factor that I really care about is autolysis, as this will create formaldehyde flavor in the beer. For modern high quality yeasts you really don't have to worry about this for eight months. I use this as an outside guide, as I will not reuse any yeast over eight months. Generally I reuse my yeast within two to three months, as that is about my brewing frequency these days, and I can say that I have never had this problem.
There are only two more factors to consider, contamination and start-up. For contamination you just want to make sure that your storage vessel is sterile. I use a pint sized mason jar. For start-up you want to get enough yeast at the right time in their life cycle. I never use a starter.
When I transfer from the primary to the secondary I harvest. I try to leave just a little beer on top of the yeast cake then swirl gently. If I do it right when I put the jar in the fridge about half will be visible yeast and half will clarify. I try to get it before fermentation has completely stopped but has almost stopped. This leaves a little sugar in the beer. The cold in the fridge crashes the yeast.
On the day of brewing I pull out the yeast before I start warming water and open it, then reseal it and shake and I allow this to slowly come up to room temperature while I brew.
After I cool down the wart I will transfer it to 5 gallon buckets. I will pitch the yeast in one of these buckets before siphoning into my fermenter.
I generally make about 10 gallons.
Well, I don't say that I do this "right" but I have never had a problem, and I have done at least 20 batches this way.
Hope this helps!