Any holes will let grain out. I just replaced mine after about 10-12 batches when I noticed one almost as big as the end of my little finger. I thought about "darning" it, but there were quite a few little holes.
When I get a fair sized hole (about 1/4" or larger), I clean and save the bag. When I get two, I use them by doubling them up so that the holes do not line up. Works fine for a number of additional brews.
It might have something to do with using a 22 inch stainless steel spoon as a mash paddle in my case. I need to make a wooden one. Anyway, I've only had to replace one bag after about 12 batches.
I'm about 55 batches in on the voile bag a friend sewed me. I just brought it back to her to get a couple little holes sewed up. They weren't on seams -- not quite sure how they happened.