Service Factor (SF) is a concept unique to NEMA design motors and is defined as a percentage by which the motor can withstand an increased load while still maintaining rated torque and speed within an accepted tolerance, even though current is expected to rise.
This should be a short time rating. You will never find an amount of time discussed, mostly because it is unclear and vague. When using a motor's service factor, an increased temperature rise is to be expected and therefore the motor life will be reduced.
Many OEMs choose to view service factor as increased power at the shaft, mostly in order to avoid stepping up to the next size motor if their application calls for slightly more than a standard motor design allows.
In any event, use a service factor with great care. Its existence will not increase available motor torque, nor permit more frequent or more severe starting.
Service Factor became a marketing game between manufacturers. Sleight-of-hand, smoke & mirrors to make the customer think he is getting more and not paying for it. It is all in the insulation Class and temperature rise.
A Class A motor at Class A temperature, or B at B, or F at F, even H at H will last the same amount of time. It is when the motor is run at a temperature below it's top most rating that "extra" life is experienced (assuming proper maintenance & lubrication). Most everyone uses 2x the life for each 10°C reduction.
So, service factor only accommodates bad performance by not freaking out when terrible things happen (voltage or current imbalance, VFDs, etc.).