Quite simple. What is your landing date? Let’s say it was January 2, 2019. Count to 5 months after which is June 2, 2019. I would always add a couple of days just to be sure. Also if you will be away from Canada for a long time anyway, you still may not be eligible for OHIP. Call them and report that you will be away and for how long to be sure. Else, you risk paying back all costs if you use OHIP when ineligible.