If you reload like I do and process in batches, then I'd suggest taking the number you want to shoot per week and triple it. So if you want to shoot 10 rounds a week, then 30 cases will do you. I always have a batch just back from the range in the tumbler. Another batch being sized and trimmed. And then another being loaded.
Of course you'll always be picking up brass, or looking for it on sale. As the brass approaches EOL then it will start to split out. If you tightly control your batches then the whole lot of 10 could split out in the same range session. If you are always introducing range pickups and not that picky about heritage, then culls will be a continuous 1 or 2 per range session.
Life per case is dependent upon numerous variables, but the main one is the stress of the load. Hot load = Short life.