i currently have 3 legacys, in the sims 2, 3 and 4. in ts2 i'm currently on generation 3 and have been playing since the beginning of last year.
my sims 3 legacy is my most favorite one!! i've been playing it since september 2013 and am only on the fourth generation (although some other family members are on their fifth generation.
i had two in the sims 4 but kind of gave up on one. that one i believe was four generations and i started that when the sims 4 came out. the one i currently am playing i started in april last year and am on the third generation, starting the fourth one very shortly!
i take a long time with my legacys but they're fun :)