I just went hunting through my back account and my Google Play account and managed to find the last six times I bought diamonds, each time with the 100% offer, and they were more unevenly spaced out than I thought I remembered. I think this was all of them, since I did not spend any money on the game for at least the first year I played.
Anyway, the oldest I found was Sept 2017, then 7 months later in Apr 2018, then months later in Aug 2018, then 8 months later in May 2019, then 4 months later in Sept 2019, then 5 months later in Mar 2020. Once I decided to start spending a little on the game, I still did not use the offer every single time I got it, so that probably explains the 7 month and 8 month gaps in purchases. That would mean I have been going between 3 and 5 months in between 100% offers.
Also, every time I bought diamonds, it was for the smallest deal, except for the one last month. I spent $10 on that one, instead of $5. And now I wonder if my increase in amount in any way accounts for the quick succession of offers I have gotten since then.