60 - 90 minutes with absolutely no degree of consistency. Sometimes I get bored within 10 minutes, sometimes I can play for 6-7hrs straight with only brief breaks to replenish beer.

I am more likely to play for hours on console than I am on PC though, if that helps at all. Something about dossing on the sofa with only a few buttons to press instead of having to remember what buttons I bound everything to and having to tweak settings because my PC is utter pants just appeals to me more.

Also if I haven't played in so long that I can't remember any of the buttons, people coming in and going "What are you doing, you need to press X/Y/RT/whatever to do that!" is likely to cause immediate cessation of any gaming urges I may have had (*cough*digibob/RR2*cough*)

I'm very happy with 10-15 minute intervals at which point I can quit without being a lamer. Any longer and I'm much less likely to want to play another round.

I prefer quicker matches than drawn out ones, for objective games I think RtCW had the formula right by giving each map the timer they needed rather than everything conforming to a blanket 20mins per map... Beach was 8mins, Depot was 15mins, Sub was 10mins etc. Forcing maps to a set regime can cause them to be stagnant... if a map is looking to be good then make it good, don't ruin it with rules.

60-90 btw

Usually 1-2 hours per game, and then I move to another game for another similar session. @ games of LoL and then 1-2 hours of tribes is my normal evening

I usually stick to one MP game, and dedicate my time learning that.
So in the beginning - it could be 3 hours, assuming ofcourse theres plenty of depth.
For SC2 which I only started learning to play online since xmas, I can go back to back to back 40 minute games quite easily. For ETQW, in the beginning - 2 full campaigns back to back were quite easy to do, this however decreased to 1 campaign, and later on 1 map.

When the dust has settled and theres no learning or experimenting with the game, if its a brilliant game 60 to 90 minutes.

I knew quite a few players who would play ETQW more for the social aspect, rather than to play the game. Ofcourse they never would admit such a thing, but you knew they didnt really care about the game so much.
Voip in-game whether it be team or fireteam voip, as well as ventrillo, was an easy convenient way for these people to socialise during the week. As long as they knew that was available to them, they would consistently "play the game" for extended periods.

Depends on game, on team. I've used to play ET for 2+ hours, played ET:QW demo/full game for hours when it came out, i can't play BRINK for long, most of the time one one map, sometimes two. Now when i have more free time i could say that i play mp games for 1-2 hours a day, sometimes even more.

They've been asking questions for ages now.

Roughly 7 seconds

I get headaches when play for too long but sometimes if having fun can be like a good couple of hours.

Not sure I understand if you're asking for map round time or multi round/map session.

For round times, I'd prefer to play like in RtCW -where maps had default time ranging 8-15 minutes AND also a possibility for server admins to set whatever round time they want per map.

With "reasonably" short round times you can get some multi round/map carnage even if you only have an hour to spend before dinner.

However if the question is about session times rather than round times, then I'd say I play more than an hour per session.

For incitament to play longer sessions, I'd say clanmates around and well admined servers (for which the server tools to do that has been in general sadly reduced in newer games).