Currently, scripts are sold with no specific time-frame. You are purchasing the script for "the lifetime of the script". There are obviously unwritten rules here for scripters such that we cannot simply abandon scripts after a few months, but of course many of us eventually pull our scripts from the store for various reasons.
I've long been a proponent of monthly pricing on scripts so that consumers know exactly what they're paying for, as lifetime licenses are not realistic. It's an interesting problem with many possible solutions.
Sorry about your old scripts not working.