How long is starter plan available?

It will be available as long as the limits(monthly submissions, or payments, or form views) are not reached, if they are reached, you will need to wait for the counters to reset on the first day of the following month to have the forms available to be used.

If the upload space limit is reached, you will need to upgrade to a paid plan that provides more space, or delete submissions that contain uploaded files in order to free up space.