I don't know exactly how long the eggs can be expected to survive if she is not sitting on them, but my guess would be 2-3 hours, maybe a little longer if it is relatively warm where they are. If they have been left alone longer than that at any point, I don't think they'll hatch, even if she is sitting on them now.
I can't say exactly what your hen will do, but mine would care for the chicks they already have, rather than continue laying on something that might not even be able to survive if it did hatch. Evolutionary speaking, I guess staying around the nest too long is bad - both because wet chicks, broken egg shells and possibly rotten eggs or dead chicks might smell and draw the attention of predators, and because the vicinity of the nest in the wild might not have enough food for the chicks that already hatched. So to avoid placing them at risk, it is best to leave the nest as soon as possible. Any chicks hatching late might do so due to being weak or slow to develop anyway, and even if they did hatch and prove to be as strong as the others, being a day or more behind the others would cause them to be much slower.
Anyway, last time I found a wet chick in the nest when the hen had left, it was still alive and I put it under a lamp to heat it. When it was fully dry and had had a little to drink, I put it back under the parents. No chicks died the next day, but the day after that, one chick died. As they were all wild colored, I can't tell for sure, but I am convinced it was the late hatcher that couldn't keep up.