@Michael502. Yep, that's tricky, but you can designate in your Availability settings which period of time you only allow 90 night stays. But, that would depend on whether the 90 nights would cross over into 2018 where you have bookings in, say January.
From reading the article, my guess is that you would have to do the following in this example:
You reach your limit for 2017 on November 15 but have short term stays booked in January so you couldn't require a 90 night minimum because you have reservation conflicts. In that case I suppose you would have to block all the remaining nights in 2017.
If guests are booking for 2018 and you reach your limit then you would have to change your availability to minimum stays of 90 nights assuming of course that there are no conflicts with other reservations. For example, let's say you have bookings in January, March, May and a booking in July that takes you up to your limit. You would then need to change your minimum stay to 90 nights beginning in July after the confirmed reservation as well as block dates in between the 2018 reservations. (Ugh! So glad I don't have this problem in California!!)
To set a minimum stay requirement for a specific date range, go to your Availability setting and Edit Trip Length. You will then see "Add a custom rule for seasons or specific dates" where you can enter a date range for the 90 night minimum to be in effect. In the example above you would enter the date after the last reservation in July with an end date of January 1, 2018.
Does this make sense to you?