a little confused by what you're asking but Ill give it a shot. These are just what I do, Im sure others are different.
If you're going to sand and reclear universal. I don't personally ever go below 800. When doing our projects and going in for the 2nd round flowcoat of 3 more coats of u.v or euro, well block it down with 800 or 1200 and than clear it.
If you've already sprayed universal and looking to prep if for 2 tone, minimum 24 hours at 70 degrees, depending on how long you waited before clearing over your original base you might want to wait longer. Also with what color you're putting on top of the uv, you might want to wait longer to spray on top of it to avoid shrinkage or any solvent interaction that might cause a problem.
how long before you have to recoat, if you will be sanding it down and at that point you go about your normal prep process. If you're asking how long before you can clear over your base/ whatever clear didnt get color on it, that you'll have to look at your manufacturer of base tech sheet. With most spi stuff if it has sat for more than 12 hours after original prep process its always a good idea to hit it real quick with a scuff pad just to be safe.