What are the factors that significantly affect the life span of a DC motor?
Good bearings and brushes, preferably self lubricating

voltage equates to speed of rotation - more volts, faster rotation

current equates to torque. More load, bigger current.

Explanation

Basically the speed at which the motor turns induces a back e.m.f. just under the value of the applied voltage. The actual speed depends upon the type of motor but for small DC motors a rough guide is about 1000 r.p.m. per volt

The current that flows through the motor is determined by the difference in these two voltages and the coil resistance. 

Adding a load on the motor's shaft (torque) slows it down and increases the voltage drop across the windings as the back e.m.f. is reduced. This increases the current taken by the motor.