Lonewolf-
A trick I picked up is to take a piece of wire, and bend a small "L" shape at one end, Run it up and down inside a fired case, in the head (base) area. If you feel a "catch" slightly up on the body, in the head area, it usually means the case has thinned, and a possible case head separation is near. There is no set number of reloads to expect from a case, as it varies with brand, action type, pressures, and a ton of other variables. You will get longer case life if you use only brass fired in that rifle, and neck size only, provided the rifle in question will reliably chamber and extract the round.
Larry