Consider the amount of use to decide how long a vacuum pump can run without any risk. Usually, it can run continuously for about 12 to 48 hours for evacuating 500 microns air. If it hangs around 15 inches, there might be a leak. Also, one should inspect the color of the oil before making it run longer. If the color appears to be white, then it means that the system contains more moisture, and it’s time to replace the oil with a fresh one in order to achieve good results.