I am using hashcat on a laptop with Nvidia 960m GPU. Everything is good. Monitoring the GPU temperature I noticed it doesn't go above 73c. Which from reading around it sounds like normal under heavy usage. My question how long can I put the GPU under such utilization before i can cause harm to it. Its utilizing 99% of its processing power
Looking at the NVIDIA web page for the 960m, there doesn't appear to be a listed max operating temperature. From some light Googling, it would seem that 73c is perfectly normal. You'll want to start worrying when you're running at 90-100c, especially on battery power, when the system throttles the device to save power.
As Techie007 has mentioned, you can run the card until it rusts at 73c.