Sometime is not a specific timeframe in the future. For example, say that your friend has done a favor to you and you want to thank her. You say to her, 'Thanks for your help. I hope I am able to return the favor sometime.' On the other hand, some time is specific. You reassure your friend who is jittery about a test that she has to take. You say to her, 'You have some time before your test. So don't worry. You will be able to do a revision of all the lessons.'