How long a cassette will last is difficult to answer. If I find a scratched negative or a light leak, I put a small red self-adhesive dot on it. If I get the same bad results with that cassette a second time, I discard the cassette. If I do reuse that cassette, I'm careful not to use that cassette for "good" pictures. To protect the light seal and reduce the risk of the end coming off, I store the cassettes in the plastic containers commercial film comes in. The cassette is either in the camera or in the container. I find camera stores are happy to give the containers away. BTW, I prefer plastic screw tops over the metal clip type because (a) statistically () they have a 50% less chance of popping open (1 versus 2 end caps), and (b) I have never had a plastic cap accidentally come off, whereas I can't count the number of times the metal caps have popped off).