everyone has many valid points on this. My Team's stance has been this:
We'd like you to go out for roughly an hour. Stay in as long as you're comfortable. We won't tell you when to come in, you tell us when you need to pit/get out/gas/etc.
Fatigue plays a HUGE factor. Think that OEM seat is going to be more comfy on the track than that $400 race seat you passed up on? THINK AGAIN.
Make sure you have water on-board. bottles, a camelbak, roof-mounted siphon tank? use it. you want to minimize the amount of work someone in the hot seat HAS to do. make sure you're hydrated BEFORE you're in the car. Jay often says "If you don't need to pee when you're getting into the car, you haven't drank enough" I tweak this a little. usually i hold out, until within 20 minutes of when i expect the need to rotate, pee, then keep drinking until they're strapping me in. Works great.
@ Nelson Ledges, I did a night stint from around 10:30-4am. I had to stop every 2 hours for fuel, so I got out, got to stretch, and assess my fatigue level while the seat & Harness weren't stabilizing me. I got out of the car @ 4am for fuel, and mentally I was terrific, but my body was starting to not cooperate so much in getting me out of the car. Definitely time to change drivers at that point, no matter how fresh my brain feels about it. I'm a fat guy, but I have 4 years of kart racing, and a total 5 Lemons races under my belt. I am the opposite of a newbie at this, so DO NOT use my personal experience as "THE BAR" to which you hold your teammates or yourself. Nelson Ledges is a gentle track by everyone's account, the weather was cool, and, I drove a Buick with stock suspension of all things not belonging on a track. Basically, I couldn't have asked for a more comfortable setup. Even in 40F temps I still was consuming about 1.5L of water an hour, plus I was in the car for those LONG cautions, and for the midnight redflag/temporary track shutdown. Finally, I have mentally prepared myself for the task at hand. I've sat in the car to simply meditate, I've watched videos of other people driving the track, and I took great care to stay out of the thick of things whenever reasonably possible.