Depending on the character, I usually keep my P1 and P2s alive until 22 or 23 (assuming no or minimal pausing). This is when they die off even with perfect care for their whole life time. If it is possible to keep them longer than that, I haven't heard of it.
All of my Morinos have left at about age 9 or 10. Can't seem to keep them longer than that.
My last Futagotenshi on my Angel lived to 15 years, and I still have a Ginjirotenshi right now at age 15. Angels are SO needy in old age.
As far as generations, I think the most I ever had was 26 on one of my V2s. Now, I don't stay interested longer than 2 or 3 generations so I'll probably never get up there again. That was back when I was SUPER into tamas.