Studies have shown that - after adjusting for your class averages - the amount of time you spend studying doesn't really matter.
A lot of students plan on 4-5 weeks. If you feel like you're starting to burn out then move it up and take it at your peak rather than completely spent.
I think I studied for about 8 weeks. I peaked around 5 or 6, and then I forgot some things and then I crammed right before the test. It all worked out, though.
You can find the studies quickly by going to pubmed and searching for 'usmle step 1' but essentially they performed a regression analysis to adjust step 1 score to people's grades (ie, they "put it on a curve" so that people who took it earlier but had great class scores had their step 1 lowered, and people with with poor class scores had their actual scores raised). [disclaimer, this is not actually what a regression analysis does, but its purpose is to acocunt for possible confounders. in this case, prior knowledge/performance].