You can start to apply for it now. You can decide

when you want to use it, for example, right after your graduation or a few weeks after that. Unless you know you will have a job right away, you can have the starting date of OPT to be a couple of months after your graduation date. No matter when you start it, you will have a year from the starting date. When I did mine, I had it start late giving me some time to visit China. Good luck