A lot depends on your batting. I usually do 2" for my bindings, but it might not be enough for a thicker batting.
Your best bet is to make a sample quilt sandwich using your batting, cut a 2" strip, then apply it and see how it looks. If it's too difficult to turn the binding to the wrong side to cover up the machine stitching, then you may want to cut the strips a little wider.
Incidentally, I recommend ironing the binding away from the quilt body before turning; makes turning much easier and the binding more even on the wrong side!