The coin boost count in time you run map not duration.
When you reach count limit boost will worn out even timer still running.

During Febuary I always left 15 mins+ on boost duration but boost effect already gone.
After that I never use it, just only run normal map to collect coin.

PS. During our current event the busty stamp rally or what ever it's name I don't care.
On DMM they do held coin boost campaign for 1 week.
You will get more coin from every map you run 1.5 time from normal.
But no something like that on Nutaku since it's Namely server hahaha.