You can obtain RPM speed by measuring encoder channel A frequency with FreqMeter component, see #28 of the discussion:
This method is very accurate (~1ppm) and has large dynamic range, but not very fast (as shown, the minimum time base was set to 50ms), which is OK for motor with large inertia. This approach can be expanded further to measure quadrature signals (A+B) if really necessary.
It is sufficient to measure signal from a single channel (A or B) if direction of rotation is always the same, and the encoder has no noise. At 256 counts per revolution, the encoder is likely the optical one with clean output.
I've had experience in a project similar to what you describe. I used Hall sensors configured as quadrature output on a motor to determine relative speed but more importantly for my application: Position.
If you're not looking for direction, theoretically you only need 'A' input. Input 'B' only gives you twice as many counts in the same revolution. If you're looking to use it for positioning data, you will need 'B' to be 2x more precise.
\odissey1 is correct, the link will help to determine frequency fairly accurately. However, I've never known a motor to be highly stable frequency-wise. Due to many factors including fluctuating system voltage, over- or under-sensitive motor control circuits and cogging-torque, the movement of the motor (and hence the output of the quadrature outputs) will have a fluctuating frequency. Therefore having a very accurate frequency measurement will be futile.
If you do need frequency, the easier/quicker method is to create a table (or formula) where the counter ticks between quadrature pulses equal a frequency value. Using a counter with an input clock at least 10x faster than the maximum pulse frequency will give to a reasonable frequency table. If more accuracy is needed, you need to increase the counter frequency (and possibly the # of bits of the counter).
If speed of the motor needed, you can create a table (or formula) where the counter ticks between pulses equal a speed value.
Similarly, if positional data is needed, a table or formula with a 0% to 100% travel (assuming a motor is driving a rotational-to-linear mechanism) can be effectively used. Of course since the quadrature signals are only relative based signals, you will need some reasonable determination of the 0% and 100% reference points to translate the relative sensor to be absolute in your system.