I have a very slow internet (120kbps) and so even saving a small file takes 15 minutes on a good day. I read tor switches circuit every 10 minutes, but if download proceeds more than 10 minutes, does it keep using same circuit without changing new? Thanks.
Tor should keep the used circuit around until it isn't used anymore. (Aka your download finished.) Of course it can happen that a circuit dies even when it is in use. You should be able to resume a download successfully in most cases. Checking download signatures is advised provided they are available.