They send one of these "offers" a week. A day, two days, maybe three days to book. I cannot imagine it is very effective because you need a real coalescing of your plans or willingness to commit to a trip with their short deadlines for these promotions.
They never seem to offer the discount on Plus Flexible which is pretty dumb. The fare could be $1,000 more (say YVR or SJO or LIR) so they worry about another $200 discount where they'd keep another $800 of the fare, vs. putting a person in the same seat on the same flight and earning 80% of 1/2 the base fare. THe only risk to them is someone cancels or changes the date, hardly a huge risk.
I have an alliance arrangement with 15% off all their fares all the time so this is even more pointless.