I don't think there's any limit, really. As long as he enjoys it.
There are other ways to massage the prostate without having to enter through the anus. If you have him on his stomach, you can stimulate it by pressing firmly on his butt cheeks around his anus (especially closer to his penis). You can also firmly massage in between his legs when he's erect, and you can get some stimulation. I might add, though, that sometimes either of these techniques might not work. It's touch and go depending on the guy. It's worth a shot, though. Just make sure you push hard, but not hard enough to hurt him!