Since Guardian's Aspect would regenerate at a rate of 20 per real life day (I think?), and the cap was 200, I'm gonna say that the rested XP cap is 10 days worth of resting.

Just my guess, based on the old system.

A better question - when is the first "tick" of rested XP accumulated? I'm guessing 1 hour in the room at minimum, perhaps half hour though. Alternatively, it could be connected to the in-game days.