I'm a bit curious what brought this question on. A bike has the clearance that it was made with. If you are getting a frame built, tell them 20mm of clearance, unless it's going to be used a lot on gravel, in which case tell them 30mm
I don't think I'm the only builder that has aimed for 10mm and gotten less. It's okay, but I hate the sound of gravel scraping the inside of the fender.