Some basic questions concerning Stepper motor with geared head/geared box
First thing first, reduction of 1:56 means that for every 56 turns the end shaft of the gear box will do 1 turn. The more gear reduction you have the slower the motor will turn but with higher torque.

When using a gearbox you are trading motor speed for torque. It is quite simple calculating the RPM from the reduction ratio. Assuming you motor spins at 500 RPM and your gear is 1:46 then the motor with gear will spin at 500/46 = 10.86RPM.
In stepper motors it is not so simple to determine what is the motor top speed as it depends on the voltage you use, the driver (better drivers can drive the motor faster) and the quality of the motor. It is also important to know that stepper motors have different torques in different RPMS and "Usually" (but not always) the slower you go the more torque you have.

Gearbox does not affect the motor life no mater what is the gear ratio. Each gearbox has a torque limit and applying a torque higher than the maximum recommended torque will damage the gearbox