As long as it takes to build to 50 I am going to guess the water level is 10-15' or more? If that is the case that pump will only deliver about 4-5 GPM, which is why it won't run two sprinklers. With a check valve on top of the point well you can lose prime if the vacuum is lost below the check valve. Either there is a suction leak before the check valve or the sand point is not sealed to the earth above the water level.
Yes pumps are made to run continuously. It will say "duty" "Cont" for continuous duty right on the side of the motor. I have a submersible that hasn't shut off in 18 years. They like running 24/7. A jet pump is the same except the ball bearings have a certain life span, but it is figured in years of running.