Look on the nameplate and see what the "duty" rating is. Then look for duty ratings at NEMA.

P. S. They are not free.

Duty rating refers to the length of time a motor is expected to operate under full load. The motor may be rated as either Continuous duty or Intermittent duty. Continuous duty rated motors are rated to be run continuously without any damage or reduction in life of the motor. General purpose motors will normally be rated for continuous duty.

Intermittent duty motors are rated to be run continuously only for short time periods and then must be allowed to stop and cool before restarting. It is usually possible to reduce the size, weight, and cost of a motor by purchasing an intermittent duty motor. Intermittent duty motors are available with maximum operating times of 5, 15, 30, or 60 minute duties or times.

These motors are sometimes used on devices like garbage disposals or air compressors where the motor operates for a short period and shuts off after the job is accomplished and will not be needed for an extended time period.

Copied from the web.

It's not quite as simplistic as a look-up table, but almost. There is a load factor involved and you must take the lesser value of the look-up table OR the load factor and duty calculation. The NEMA document explains it in detail.

Although the official NEMA document (NEMA MG10-2110 Table 7) would cost money to purchase from NEMA, that specific information is not guarded and in fact, NEMA uses that table within other free publications. Here is one. The information you seek starts on page 18.

Oh and by the way, an Engine involves conversion of a fuel source to kinetic energy. You are referring to a Motor, which converts one form of energy, such as electricity, to kinetic energy.

Too many starts in a short period of time - especially full or heavy load starts will incur very heavy demand charges from your local utility. Edmund

Being that he asked about NEMA ratings, he is likely in North America. That would not likely be a factor in demand charges for 99% of the utilities here.

Peak demand power meters are in use around here, and the customers are charged for all peak demands beyond their contract allowance. Read some of the fine print in electrical utility contracts for industrial plants.

The types of intermittent operation of an electrical motor are quantified in the IEC definition of duty cycles. How you operate a motor will fall under one (or more) of the described cycles. If a particular motor has been sold as for a specific duty cycle type, it can be used without damage at any less-stressing cycle.

Please see

http://www.engineeringtoolbox.com/iec-duty-cucles-d_739.html

http://books.google.com/books?id=s1eNoZsEPvsC&pg=PA326&lpg=PA326&dq=duty+cycles+of+electrical+motors&source=bl&ots=xW6YC1f7hM&sig=JYjGR7IVIIMRHq0J8ok4_VVXhC0&hl=en&ei=kvBMSs2TIs2etgfFyu2iBA&sa=X&oi=book_result&ct=result&resnum=10

When I was doing work in power stations, most of their larger motors (480 - 1,000 volts and 100 - 2,000 HP) were rated for 2 start attempts in a 10 minute time period. If these attempts were not successful, then a 60 minute cool-down period was called for before the next attempt. This goes back 25 years so it may not be dead-on, but it's pretty close.