Heat dissipation in stepper motor running for long time
If you're operating the motor within spec, you should assume that it will be happy. There are exceptions, since some devices are only rated for intermittent use. Examples are solenoids, which are often not rated for continuous use, and cheap laser modules from China on eBay. Motors, though, are generally good. Especially, try to mount the motor to a metal plate if you're worried. Alternatively, just take a motor, power it up, and leave it running for a day on a nice fireproof surface and see what happens. If you can touch it without raising a blister you should be fine. Come on, where's your sense of adventure?