You only borrow points when making a reservation that you don't currently have enough points in the use year needed for the reservation. Example, you making a reservation for a stay at OKW that needs 150 points for next May, and you only have 140 available at that time. You would need to borrow 10 points from the following use year to secure the reservation. Remember though, once you borrow the points, you can not put them back into the orginal use year if you have to cancel the reservation. Hope this makes sense and helps.
Because borrowing isn't a stand-alone transaction like banking, there's no deadline associated with it. The timeframes come into play, instead, with the booking rules for the vacation that will use the borrowed points.