The Pending Posts limit changes according to the Postcron plan you're on and determines the number of scheduled posts you can have at a time. This means that it's not time-based (defined by days, weeks, or months) but rather the total of scheduled posts you can have across all your social accounts.
For example, if you have a Personal plan, your pending posts limit is 100. This means that you can have a maximum of 100 queued posts scheduled to all of your social accounts at the same time. To schedule more posts, you need to upgrade to a higher plan.