Depends on how much money you want to spend. And how it's going to be used.
If it's rebuilt with good qualitly parts, and by someone who knows what they are doing, then they can last a decent ammount of time.
What do you plan on doing with the rig? Rocks, Mud, DD, Trails? What size tire? How much power? Even what kind of rig. A buggy will be a lot easier on a Th350 than a Suburban.
I plan on putting a Th350 behind the 454 in my Camaro next summer. But that only see drag strip time.