"Long enough" is the short answer. Long enough for how many hundreds of thousands (or millions) of men, women and children to walk the distance, plus all their many flocks and herds. Long enough to get every last one of them safely on to the far side.
The long answer (gleaned from the account in Exodus chapter 14) provides important details. Such that it was during the night that the pillar of cloud that had been leading the way to that point where they camped "near Pi Hahiroth, between Migdol and the sea" (vs. 2) moved away from the front to the back of the company. This blocked Pharaoh's army from getting any closer:
"The pillar of cloud also moved from in front and stood behind them,
coming between the armies of Egypt and Israel. Throughout the night
the cloud brought darkness to the one side and light to the other; so
neither went near the other all night long." (vss. 19-20)
Also during that night "Moses stretched out his hand over the sea, and all that night the Lord drove the sea back with a strong east wind and turned it into dry land. (vs. 21)
That takes up the events of the entire night. It is surely right to say that such divine action while Moses' hand was stretched out is a vital part of the crossing of the sea. After all, without it, the company could never have even started to cross. Your comment implies that you take the night to be the time period and this information in Exodus 13:21 shows that the crossing could have commenced during the night:
"By day the Lord went ahead of them in a pillar of cloud to guide them
on their way and by night in a pillar of fire to give them light, so
that they could travel by day or night."
After all, nobody was going to get much sleep that night anyway - what with the army at their back though blocked by the pillar of cloud coming between them, Moses standing with arm outstretched while a fierce, supernatural wind blasted in, and the pillar of fire at the front, showing the parting of the waters into two walls while the sea-bed dried out sufficiently to take the weight of that huge gathering. The pillar of fire moving ahead was their signal to start marching between the walls of water. And that could have begun during the night. But the Bible does not say exactly when the march began, nor when it finished. It could well have started during the night, continued through dawn and gone on for as many hours of daylight as was needed to get them all safely across. The pillar of cloud at their back continued to prevent the Egyptian army from getting close - until God removed it so that they would career into the still-opened channel, and then discover it was now their watery grave.
Given that it's unlikely the army would have ventured into that channel during hours of darkness (no pillar of flame for them), I would assume the same day that the nation completed the crossing, in daylight, would be the day of that army's total defeat. But the account adds that “it came to pass that in the morning watch” the Lord began to dismantle the chariot wheels and the army became bogged down, unable to go back to the dry land they had come from (vs. 24). We might assume that that was the same morning, but what if it was the following morning before the crossing was completed? It would also help if we knew how broad the dry channel was, but we are not told, so those are unknowns.
For sure, night-time was no problem for commencing the crossing, given the pillar of light at their front, and the awesome sight would go on for as many hours as was required until that “morning watch” when God began to trouble the army in the midst of the still opened sea.