Mining itself won't hurt your card as long as you are doing it properly, which I imagine you are. What could hurt your card is if other hardware is insufficient. For example, a high temperature for a long time. I would say that 76 C seems a little hot to be running at for 8 hours straight every day, but I think it is acceptable. I generally like to keep my 770 below 70C, even if the duration of load is only an hour or two, but that is personal preference. Some people run their cards at 80 or higher C for 12 hours a day, so you might be fine, but then again those are usually people with AMD cards, which usually run hotter and are built for it.
Also, if something happens during the day, like a power outage or short, that could potentially harm your GPU, but there is a low likelyhood of that happening.