I read where David (Hobbes) mentioned that a cake is good for 30 sessions. Initially, I thought he was nuts, but I seem to be using more and more leaf as time goes on. I used to think that I would get 50-70 sessions from a standard 357g bing, but upon further reflection, I believe the number is closer to 35. I tend to discard the "shake" (let's see how old you are) and concentrate on using chunks. Pu'er seems to require more product per session than other teas.
I've been getting 10-12 sessions per 100g tuo, and a bit mor than two dozen from a 250 tuo or brick, so, for me at least, I have to figure on 10g per session, factoring in waste, when deciding on how much to buy.
How long can you stretch a bing?