Back piece

Im scheduled to start the first session of my back piece in 2 days. How many hours do you think is good for the 1st session? This will be my 8th tattoo but the biggest and first on my back. I dont want to bite off more than i can chew. I kinda want to maybe do 4 hours and see if I can handle it. What do you guys think about 4 hour session? How many hours does a artist usually prefer? My artist hasn't gotten back to me yet but I just wanted different opinions.