Duty Type of Universal Motor

If designed for continuous duty operation they handle it just fine same as any other motor designed for continuous duty.

IEC duty cycles of operating electrical motors - Engineering

Universal motors are those motors which are designed to operate at both AC and and DC and also can be easily used in both forward and reverse direction so they can'y be of continuous duty type

Why does the ability to reverse or operate on AC or DC affect the duty rating of the motor? From Sri Balaji Motors: "...This motor is a AC / DC universal motor, with high starting torque and continuous duty capability..."

Now if you said that it was a very high speed application and that it could only be run for 30 minutes or less above 15,000rpm before it overheated, then I would say "ok, speed affects duty cycle". The reality is that all of the operating conditions affect duty cycle such that the same motor in one application might be fine with continuous duty but not in a more challenging situation.