There are as many different answers to that question as there are quilters who use it.
The first time I used it I went fairly heavy and it did not move. One of the ladies I took a class from at my LQS store was talking about pinning versus spraying & her answer was you just Spritz and she mad a zig-zag with her hand. I tried the zig-zag and found that there was not enough to hold.
My answer to your question is: spray lightly and test the fabric, if you find it is not sticking to your satisfaction, then spray LIGHTLY again. I tend to hit the corners a litle heavier, but I try to not over do as it can be a little hard on the breathing and you do not want so much that you gunk up your needle and machine.
Happy Quilting
Susan.