How much work can be done by a 300 w motor in 60 sec?

IF the motor is totally loaded to its full rated capacity, THEN

it's doing work at the rate of 300 watts = 300 joules per second.

If it can keep this up for 60 seconds without overheating, then

the total work done is

(300 joules/second) x (60 seconds) = 18,000 joules