[Alan Anderson]

For that application, you don't really need to turn the motor a definite number of rotations. You can either choose a motor that can withstand being stalled for a few seconds and drive it for a long enough time that your mechanism will reach a hard limit, or you can use a sensor that detects when the mechanism is fully retracted or fully extended to let the motor run only as long as it needs to.

There are also a couple of nifty mechanical tricks you can use to let the motor keep driving for a while without stalling, like using a lead screw with unthreaded ends and a spring-loaded stage that will be certain to reacquire the threads when the screw turns the other direction.