Some people could fascinate me for hours in a day. But every 75 minutes, I need to go visit the room named after my gender. From the line outside the room at the local symphony concerts, this is not an uncommon need.
I could go 8 hours at a time as a teenager without plumbing, but being forced into the Army changed all that. People in shape drink more, and visit the room more often. I decided to stay in aerobic shape as a result of the various studies in the medical journals, and this involves drinking water etc. Not a common practice among pianophyles, I will wager, but piano is part of my exercise program. It is more aerobic than crossword puzzles or facebook.
If you decide to play several hours, provide the 15 minute breaks traditional at concerts every 60-75 minutes. If people really aren't interested, they won't come back from the break, That way the audience is not forced to insult you by walking out with the lights off while you play.