During the week, I've only gone on Fridays. I usually get there around 10 am or so. It varies week to week, but I would say that the rifle range is usually mostly full by around 11:15 or so. Sometimes it's full about 11:00 and sometimes not until right before it opens. During the weekdays, the far right/makai stalls are closed from about the soda machine to the end, so there are about 1/3 fewer benches.
I've also heard that Thursdays are usually more busy than Fridays. I've never been, so I can't tell you for myself. If you wanted to guarantee a bench on the rifle side, I would say plan on being there by 10:45 or so. I don't mind waiting and usually just pick up food on the way and relax and eat while waiting for the range to open.
I haven't gone to the bulleye pistol range that early, but in the times that I have gone to shoot pistol, there are usually a few open spots.