The clock time at which you turn around — no matter how close the summit looks. Fixed before you set off, so your ego doesn't get a vote up on the ridge.
The trick is to plan the tour backwards: when do you need to be down, how much buffer do you want, and therefore what is the latest you turn around? You fix that clock time at home, while you're still thinking soberly — not up high, where the summit looks close enough to touch and the group is pushing. The reason is mundane and lethal at once: thunderstorms tend to build as the afternoon heats up, and the descent is already the part where strength and concentration fade. Turn around too late and you'll take on the most dangerous section tired, in the worst weather, often in fading light. Say the turnaround time out loud in the group before you start. And keep in mind: turning back isn't a defeat, it's a decision you get to repeat on the next attempt.