I don’t know if your school has any practice test requirements, but I’ll put my practice test schedule from last year below:
January: Kaplan practice test #1.
March: COMBANK Practice test #1.
Mid-April: NBME 13.
Early May: UWSA 1.
Mid May: Kaplan practice test #2.
Early June: NBME 18, COMBANK #2.
Mid-June: NBME 19, COMSAE E.
Early-July: UWSA2.
2 Days before Step 1: Free 120.
It looks like I took a lot of practice tests but the Kaplan and COMBANK tests, as well as COMSAE E, were required by my school to make sure we were progressing. So in reality my goal was to take 1 per month from April - June and then every 2 weeks from June - test day, which was the 2nd week of July, for a total of 6. I took Step 1 first, then COMLEX 1 week later studying mostly OMT. If You don’t have any required practice tests you should be alright doing 1 per month starting 3.5 months from test day, then every 2 weeks once you’re 4-6 weeks out, depending on cash restrictions.
Edit: If you have any choice in the matter stick with the NBMEs and UWSAs. The COMSAE I took wasn’t horrible like most of COMBANK, but all of the questions for each subject were basically studying the same topic; All of biochem in it dealt with familial hyperlipidemias. So your score isn’t going to be very representative of your real knowledge, since if you happen to know those specific subjects really well, you’ll do great. And if you don’t, you might barely pass it.