Chicks can remain in the bactor for up to 72 hours. The yolk provides nourishment so they don't need food/water. They will, however, explore with pecking everything, including the other chicks and their own feet (making them squeal)! Mine are pretty dry and fluffy after 12 hours. The high humidity keeps them a little damp, though.
I try not to open the bator unless humidity is really high. This morning (it's midnight here so in about 9 hours) I'll remove the first few chicks. They will be around 24 hours old. I could leave them longer but I can afford to open the bator briefly (humidity high) & I like to have more visibility and less chaos for the other eggs.
I recommend 48 hours max. Preferably when you don't have pips or when the hatch is mostly over. Some won't open it at all from start to finish; but I do pull chicks occasionally and never had a problem. My incubator is verryy wet inside..
I had a problem leaving too many chicks in once. A Silkie egg with a small flat pip got turned over. When the hatch was over I realized it had drowned or suffocated. I don't believe this is common. Chicks play soccer with the other eggs all the time!