I am the instructor at Richmond, and as the only instructor my life is quite different than that of an instructor at Jacksonville, for example. They work much more, but several of them post here on JC and will hopefully post a response.
When I don't have anything scheduled, I don't go to work. If there is a long stretch without any students, I'll go in every 3-4 days and do paperwork, update my schedule, make copies, etc. When I do have a student, we arrive at 0800 and can finish between 5 and 8, depending on how much flying we do that day. When I have two students at once, I typically fly 3-4 times a day and we finish even later. You're up against the clock, and a checkride 72 hours from now.
The number of flight hours varies by month, from 4 to my current record of 45. ATP gives each instructor 5 days off over Christmas and 3 at Thanksgiving.
Overall my quality of life is good. I have a lot of time to work out and do whatever. The instructors at the larger bases can work up to 7 days a week and have little free time, but they are flying much more that I am and I will likely call them Captain the next time I see them, since it will take much longer to get to an airline at my current rate.