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Yes, sabotage CAN be the longest
But it isn't considering the slow pace of sabotage games and the 7-15 seconds waiting time between spawning

If I have a really good game then I will typically get 50 kills and 20 deaths, 20 deaths * 15 seconds = 300 seconds waiting for spawning

Thats 5 minutes of waiting to spawn
And on top of that, sabotage games have a really slow pace too

The problem with sabotage, although it has a 20 minute timer, is that some people feel the need to rush and plant it immediately, along with the spawn timer. You could get a lengthy domination going if no one captures the flags though :p

Going by all the comments above,although sabotage can be the longest,HQ infact is better,but only if you play with friends or people who know what they are doing and don't capture/finish the game in 5 minutes!

really depends what you are going for.

the longest technically can be demolition which can last like 12 hours if people continue to plant and defuse, however that only happens in boosting.
the longest starting time is sabotage, but there is a respawn timer, people often rush too and the respawn timer limits the amount of kills particularly of your air support, if you get a full 20 minute game you can still easily rack up 80 kills on a good day.
domination can last a long time if its a close game, but even a slightly 1 sided game will go fast, saying that it is very easy to rack up high kills and spawn kill with air support

hq is again a long mode if you capture it and allow the other team to capture, however unless your against a terrible team, it can be difficult to do and you could end up throwing away the lead.