From my reads on it, your body has no mechanism to counter it. So it means 200mg should still be 200mg with the exact same effect if you took it today as to have taken it on day 1 as to day 99 for example. Though there IS the half-life of 36(?) hours, so be careful of that when you're using it.
No, your body doesn't get use to it: No feedback mechanism iirc.