I've been practicing the step1(us world q/bank).How do I know when I'm ready?.

when you reach consistently percentage around 70% or more at least to be safe and go ahead and try some simulation test I would say, if between 70-80% it is very good!

It is kind of a personal preference and depends of your goal, but I think is good to have this objective to be in the safe side for your real exam, it is always better to be really ready than going in the lower limit and have the risk of failing.

after you get 70-80% at UWorld, do simulation test of Uworld and NBME, if you are getting your goal you are ready if not, take the lowest areas of performance and harm very hard on these areas and try again till you reach your goal. Take a look to this link (last paragraphs)

http://www.usmlesuccess.net/an-in-depth-look-at-what-it-takes-to-succeed-on-the-usmle-step-1