I don't think there is a standard for this. I usually find the center and measure 2.5 to 3" on either side and mark. Then I center the handles on the marks, so there is about 4 or 5 inches between, maybe a bit wider if adding shoulder straps. I usually make tote-type bags. It really depends on the bag construction and materials. If the bag is open at the top (like a tote) you don't want it to gap or bunch between the handles. A heavier fabric and/or a sturdy interfacing will allow the handles to be placed further apart. Fasteners, like a snap, will also allow wider placement. When I'm not following a pattern, I will often construct the bag as far as possible, then pin the handles on to see how the bag hangs.