To answer the second questionfirst: I don't know. I have loaded over 6000 .45 ACP rounds since March and I know I have loaded some of the cases 6 or 7 times. Found my first split one today. You can definately load them several times.
First Question about tumbling: I tumble my cases every time I shoot them. It may not be that necessary but I like the clean shiny way they look and it makes the ammo look good too. I put my tumbler in the garage and run it while I am doing what ever else I want to do. Run it for 2 to 4 hours and the cases come out looking like new.
My answer to the first question is clean your brass every time. Makes things a lot less messy when handling the stuff. More importantly it allows for better inspection of the case. Lot easier to see deformations, cracks, dings and what not when the case is nice and clean.
As for how many times you can reload the same case. That is primarily dependent on the pressures you load to, case design, and chamber dimensions. If youre loading low pressure plinking rounds youre going to get more life out of the case vs loading some super hot hunting load. If youre having to trim your cases youre probably not going to get as many reloads out of them. My 45ACP cases have gone around 6 times now and are still looking OK and dont need trimming. But 45ACP is generally considered a lw pressure round and on top of that I load mild loads for plinking. If I were loading something lke 40SW or 357Sig, I'd be seriously inspecting my brass and probably stop goign after 2 or 3 rounds just as a CYA thing.
Yup, Straight walled cases will almost last forever if yer not over crimping or loading them real hot... I get very few cracked cases on all pistol and even 45/70 cases.. Bottleneck case wont last as long..
I shoot a lot of 9mm and .357 and uaually wait till I have about 2000 empties to reload another batch.. Always run them in the case tumbler.. As was said above it's easier to spot cracks with a clean case.. They also wont hang in the chamber like a dirty one will..
Even worse with black powder loads.. They get real nasty..