A motor heats up as Amperes go through it. You need to avoid too high temperature.
Overheating can cause insulation to burn, metal to expand, solder to melt, etc.
Brushes cannot tolerate overmuch current.
The motor can drive a certain load continuously (100% duty cycle), or a greater load temporarily (partial duty cycle).
Ideally you should watch all components while testing. As a general rule keep the temperature cool enough during continuous operation so you can keep your finger on the housing.