I regularly make use of the boosts (along with spending about 5-6 FGs on stam refreshes) whenever my gold starts getting low (I started playing FKG back in March and am still evolving/leveling girls), but last week I thought I saw something odd but wasn't able to confirm it until today:

The coin boost appears to stop being applied during the last five minutes of its duration. The mission complete report will show the boost being active, but the gold amount is the same as what was collected and the gold total confirmed that the final amount wasn't boosted. I don't think it's a display issue since I was definitely still in the hour window but would welcome any other confirmation on this. I did submit a ticket just in case.