I have a 46 gallon with 3 peppermints. It took them about a week before I noticed a dent in the aptasia population, then another week and they were ALL gone. My brother has a 90 gallon and he put in 9 peppermints in there and it took them about the same amount of time. They will slowly discover the aptasia as they roam around at night. So give them some time! They do work!
How much are they for you? Here they are 1 dollar a piece, so buying 9 isnt a issue... though i've seen them for 9 dollars a piece before! So... if you're paying a lot you may be fine with just the 3 for your size tank... but just give them a couple months and whatever you do DONT feed the peppermints anything specifically, it'll make them have more of a 'aptasia sweet tooth' =c)
Enjoy.