You can check in statistics the amount of bytes that've been transfered. Checking that it's not got to 10GB in the upload total would be enough. You can download up to that point so, aMule is useful untill then. Most other P2P networks have the same problem and, anyway, using a P2P network and not sharing is, not only selfish, but also not caring about the networks live, so it is also bad for you. Limit you upload speed if needed (limitting it to 10KBps would be enough to have an unlimited download speed).
Greetings!