The assignment extension page was written for manual reservations. Users doing manual tests have no other way to tell Primenet that they are still working on the assignment.
Automatic assignments use different rules (see
https://www.mersenne.org/thresholds/)
Every time prime95 sends in a progress report, usually daily, the expiration date is set to 60 days in the future. This means that in most cases, as long as you do not take a two month hiatus, the exponent will not expire.