Just guessing but there's 64 oz in a tub and I'd guess each app would use about 3 oz of relaxer so I'd say it's good for 18-21 applications more or less.
I just bought a 4 lb tub of Mizani and used about 3 oz today.
I had a 4 lb tub of another brand before and it lasted me a long time. I never used the entire thing because I felt that it wasn't as strong and lost some of its umph. However I called Mizani and inquired about the shelf life of this 4lb tub and they said there wasn't one as long as it was stored in a cool dry place.