Well, the purpose of wet sanding is to remove a paint defect (scratch, orange peel, etc.). You would wet sand until the paint defect you're working on is removed.
Now with that being said, you have to be very careful when you do this. You want to make sure that you don't go through the clear coat or make it too thin.
You'll also have to use a series of compounds and glazes to remove the scratches induced by wet-sanding. If you're not familar with the whole process you run a risk of damaging your paint. I'd advise you to do more research on wet-sanding before you go much further.
It would also be helpful if you explained why you are wet-sanding and include pictures if possible.