Well... there is no single answer and so many questions!
I usually use some measurement from my block as a guideline, depending on the size of the project and how I decide to quilt it, I usually want at least 6" total in my border -- but obviously that is almost all of a preemie quilt and on a king sized project 6" can look wimpy.
Do you want to show off some quilting in the border or do you just want to frame the top?
So what size are your blocks? Do you have any sashing? Could you possibly post a picture here?