The way I read the rules, the mastercard is determined immediately after the second stay, and you only get one. So my read is if you stay at two holiday inns the first week of December, and an intercontinental a few weeks later, you might be getting the $50 GC as soon as you finish that second HI stay.

From the rules:
A Participant will receive the highest possible value of the Mastercard® Reward Card, depending on which IHG brands Participant stay at.
Mastercard® Reward Card value will be determined immediately following checkout of the second of the two (2) Qualified Hotel Stays (one [1] of which must be at least two [2] nights).