On the B757 there is no computer that limits roll rate, it's not a fly-by wire system, simply mechanical powered controls with both ailerons and spoilers for roll control. Max roll rate is not relevant in normal operations, one never needs more than a gentle wheel input to achieve what is needed. I can't remember using in the air more than perhaps 20 degrees of the available 90 degrees of wheel movement. On the ground, for takeoff and especially landing with strong x-wind, much more will be used.
That's perhaps a long way of saying I can't answer the question directly, but I can tell you it is speed-dependent, with higher roll rates at higher speeds. TAS or IAS? There's a question!
Incidentally, the flight manual gives no figures and no limits.