It took a month for my COS from H1 to F1 process and it was approved last week. I'm still waiting for the paper documentation, but i checked online and its been approved. I included my last 3 paystubs and supporting documents. But in my case I'm taking full time classes this summer and graduating this December, since I had been taking courses while I was in H1B.

Its not that difficult for COS if you have genuine job and have paystubs.