Minimum number of levels?

Hey, I've been working on a game and i've kinda run out of ideas for levels. Each level takes about the same amount of time to complete as a "cut the rope" level. Do you think 24 levels would be enough gameplay to make it worth 99cents?
Comments
Give your game to players to test. Find out how long it takes them to finish all levels.
(My upcoming game "Zombie Marbles" has 50 levels. I originally wanted more, but during test playing I found users took longer to complete some levels than I anticipated, and I decided to go with 50... plus I made levels easier. It was a real surprise to me.)