I have to disagree. You should start Uworld when you have a solid foundation and have already covered most of the material in class.
For example, what is the point of doing a Uworld question on leukemias in september of second year if your heme/onc class covering leukemias isn't until december? At that point, you're wasting the first pass of the uworld leukemia question since you have no base knowledge to reasonably answer it.
I suppose you could start uworld at the start of second year by system if you already covered most of that said system already. Personally, I used USMLE-Rx, FA, Pathoma, and SketchyMicro/Pharm from January-March to make sure I had a solid base of knowledge before starting Uworld. I still had plenty of time to do Uworld from the end of march until the beginning June when I took the USMLE and got 1.5x pass of Uworld.
I also had friends who started Uworld in January, hadn't reviewed First Aid, Pathoma, etc enough, and got absolutely destroyed by Uworld to the point where it demoralized them.