Fixed time limits at checkpoints and at the finish, at which you are taken out of the race. Over long distance they usually sit at roughly 2:20 h for the swim and 17 h overall.
Over a race day lasting 8 to 17 hours, the decisive skill isn't speed, it's managing time against the cutoffs. At long distance most organisers set roughly 2:20 hours for the swim, about 10:30 hours to the end of the bike leg and 17 hours overall. The mistake is checking them at the checkpoint: by then you can only react. The second is treating them as a target rather than a safety net. Experienced athletes therefore measure their splits not against the cutoff but against their own goal time with a buffer — and set three goal times: ambitious, realistic, and just-finish. What gets missed: these numbers are not a standard. They differ between organisers, and at middle distance the grid looks entirely different — race information there names around 1:10 h for the swim, 5:30 h for swim plus bike and 8:30 h overall.