It depends on the situation. If we're in a risky location, and I'm kneeling while he's standing, shorter is better. If we're comfortable, I like to go for 10 to 15, maybe 20 minutes, but if the guy is into edging or needs longer I'll take regular breaks and work him with my hands to give my jaw a rest. If I'm comfortable, kneeling on a soft carpet or pillow, or kneeling or lying on a bed, I go for as long as the guy wants it, but the connection and vibe needs to be good, otherwise I get bored.
A great suck buddy I had years ago was into edging. He would sit and I'd kneel in front of him, and he'd tap my head when he was close to cumming so I'd back off, edging him multiple times. It was a great way to give my jaw a rest while prolonging the enjoyment. We'd go for 1.5 to 2 hours and then when he was ready to cum, he wouldn't tap and I'd edge him once more to tease him. Then bring him to orgasm and it would be so intense and prolonged, with so much cum which I would gulp down. Afterward he'd just lie back and almost pass out.