There's no definite length of time. It's not exactly like 23 hours and your chick will be fine but 23 hours and 30 minutes and it'll be dead. It all depends on more factors than I'd like to think about. I'd imagine that the health and nutrition of the parent stock and how ideal the rest of the incubating conditions are will play the biggest part in deciding if your unturned egg will make it to hatch okay. Regular turning doesn't guarantee they'll hatch, it just maximises their chances. People have had reasonable hatch rates with eggs they have never turned, but I'm fairly sure that if they HAD turned them, their hatch rates would have been even better.
Turning is most important in the first 7 days, so not turning them often then will most likely have a worse effect on your hatch rate than if you turned them carefully the first week then didn't turn them regularly for the last week. But again, it's more about increased and decreased hatch rates than about them being definitely okay or definitely dead.
If you're asking because you can't turn regularly, or because you've forgotten once or twice, don't worry too much. Just turn them as often as you can or as soon as you remember and don't worry too much about it. If you're wanting to incubate regularly and know you're going to have problems either with remembering or with actually being there to do it, think about investing in an auto turner. They really do take a lot of the stress out of the whole process...