I'm gonna say start using it exactly one month before you take step 1, unless you splurge for the 3 month subscription. I wouldn't recommend the latter, I know a bunch of people who paid more for the 3 or even 6 month subscriptions thinking they'd use it long ahead of time only to realize that doing Q-bank while finishing your 2nd year classes sucks. Practically nobody used it to any significant degree ahead of time.