I thought about suggesting that one but realized, doesn't the off cycle match the on cycle? Ie., you can vary the on-off from 5 seconds to 3 minutes but if the off is as long as the on then no matter what it's set at, the pump is on 50% of the time.
I'm not sure where you could find these but I think one of these timers might fit the bill. If you find one of these can you let us know where?
repeat cycle -
http://intermatic.com/Default.asp?ac...0&cid=51&did=5
percentage cycle -
http://intermatic.com/Default.asp?ac...5&cid=51&did=5