I've been doing this for years now. I've also gone way beyond the generation recommendations with my saison yeast and i'm on like the 30th batch with that stuff now, it still tastes great. Good practice and sanitation is key.
Some tips: make sure your jar is always cleaned then sterilized before pouring in the slurry.
I generally use the yeast from the jar only once, why keep it around to use again when you could use the fresher yeast cake from the beer you just pitched into.?
Mr malty has a slurry calculator, but as a conservative estimate I always use is 1 billion cells/mL of slurry.
If the cake is older than 2 weeks, even if i am using the correct pitch of slurry, I usually 'wake up' the yeast with a 500ml starter on a stir plate, when I do this I usually have krausen in about 4-8 hours from pitching. If I dont wake up the older yeast it can take 24 hours to get to high krausen.
I recently chatted at length with a very knowledgeable 'yeast wrangler' type brewer with a cell biology background. He said the yeast in your jar naturally stratifies when it settles like in your picture: decant off the beer and the top white creamy layer, those are mutant or deviant cells. The bottom is likely mostly dead cells. The middle of your cake is those cells you want to pitch into your beer.
hope this helps it find it's a really great way to always have fresh yeast around and to keep a decent stock of strains on-hand, for free!
