@iphone6: TSC is actually faster than NSC, based on USCIS website as well as forum discussions. But, there's really no "average" time to process these applications - can vary from as little as a month to over a year - depends on a lot of things including country of birth, service center where the application went to, application itself, and the officer handling the case. Good thing for us is that there is no backlog of NIW applicants like in the case of Indian and Chinese applicants. So things should move relatively faster than the "average", but again - all those things I mentioned can affect your case.
If you don't mind, do you mind sharing some details about your application? self/lawyer petition, only I-140 or concurrent I-485, etc? Thx!