I'm a big believer in pinning, every 2" or so. So for me a 3" square would probably get two pins, one towards either side, but in general if I'm going to pin I'm going to use 3 pins at a time, so that would be a 6"-ish unit.
But... when I am doing strip piecing of WoF (width of fabric) together, I do that without pinning. After it is subcut though or there is a seam, there is pretty much always a pin. One big reason is I press open and I like to pin down the leading edge so it doesn't get flipped. I use the big long quilt pins but they are well down from the seam line/presser foot, an inch or so down.
I have friends who press to the side or don't press at all and their styles work for them. I've incorporated pinning time into a portable project that I can do at my Tuesday group, or I went to my husband's bowling league the other night, they are getting used to me sitting in the background pinning or hand sewing binding. Can also do while watching tv.