about every 4"---or each time you place your fist you touch a pin on each side (4sides)
i do try to make sure they are not in the line of stitching- so i don't have to worry about hitting them- or having to move them when i get to them- so if you have your quilting pattern drawn on the quilt top try to place the pins outside your quilting lines- i very very seldom use pins- (hate having to remove them & find them a pain to put in in the first place- if i have to baste a quilt sandwich i use the thread basting method- it is so much easier to just thread a needle & stitch huge stitches in a grid to hold the sandwich- the thread is much easier to remove after quilting and (for me at least) holds much better than pins