"Intruder is correct in saying that you can be in "knife-edge" flight, ie 90 degrees of bank and still level, in a high performance aerobatic aircraft. But you are using the rudder and fuselage for lift in this case, so in proper aerodynamic terms it does not constitute a turn, as the wings are not loaded."
Maximum:
In knife-edge flight as you describe, you can additionally use the elevator to load the wings and cause the airplane to turn.
All the equations and assertions already provided by others are likely valid for COORDINATED turns in LEVEL, CONSTANT-SPEED flight. However, other modes of flight do exist, and are used for various purposes.