Bipolar stepper motor: What should the sequence timing be?

Generally speaking, with stepper motors the issue is not that you haven't energized the winding long enough, but rather than you are starting to turn it off before you have fully managed to turn it on.

Stepper motor windings end up being fairly inductive; inductors impede rapid current rise (and fall). At fairly low mechanical speeds you quickly reach switching frequencies where the inductance dwarfs the DC coil resistance, and applying the rated supply voltage does not result in anything near rated current. The motor starts missing steps and then stalls completely.

To combat this, higher performance stepper drives are chopper current regulators which use many, many times the rated voltage of the motor. They turn fully on, and hit the winding with a large voltage step. This is impeded by the inductance, so the current does not immediately exceed safe levels, but instead begins to rise over time. The chopper monitors the actual winding current using a small sense resistance, and once the target current is achieved the drive voltage is shut off; as the current falls it may be turned back on again if the motor has not yet been advanced to a position which would change the desired current level/direction of that winding.

Actual motor properties will not be stated in the terms you seek. Rather, you may find a winding inductance spec which would help you determine what supply voltage you would need to achieve rated current within the duration of a step time at a desired RPM. You would also find a maximum winding current spec related to the possibility of damaging the permanent magnets due to excessive field, and another time-average maximum current factor related to overheating the motor, which can also damage the magnets.

Additionally, at high speeds you have to consider mechanical inertia, including of the motor itself. This means that you can't just hit a stationary motor with full speed pulses, but must accelerate it gradually. It's quite likely that if you had a motor running at high speed and stopped providing pulses it would advance many steps on its own before coming to a stop. In effect, the mechanical distance of a step is fairly trivial, but the "electrical distance" of getting the current flow through the inductive winding started/stopped/reversed is quite large. So provided you profile your acceleration and deceleration, its your ability to force that current change which will dictate your minimum step time.