If the RAM is faulty, the test will not be able to run for 7.5 hours. There is no harm in doing this, the entire point of Prime95 is to stress the system for stability in search of errors. If the program finds an error, the test will shutdown and no harm will occur. If the RAM is error-free, then it will run for 7.5 hours and you'll be fine. Just make sure the CPU has good cooling because the processor will be stressed for quite a while. When I tried overclocking my CPU, the test would shut down after 5 minutes or less if the system was unstable, and no harm was done. I would recommend using Prime95 to test for the RAM errors in addition with Memtest.