You first need to find out what the TJ max is of your Gpu processor is. You really need to do some searches on that subject. There are a lot of opinions on the web about how close to run your equipment to the TJ Max. For practical purposes, it seems that they believe running at 80% seems to be a good comprimise between performance and heat. ie. If the room temp is 0 C and the tj max is 100 c, don't go over 80 C. Quite franlkly I run at 70%.
I could see you hitting your goal of 3 years running at 70%.
If I were you though, I would do the research. The debates are actually quite interesting and there is a lot of information.