Answer:
He was in there a little more than one night - plenty of time for the lions to eat him if God had let them as they were pretty hungry as we can see when we read the story, because Daniel's accusers plus their wives and children were thrown to the lions after Daniel was vindicated and the lions finished them all off before they reached the bottom of the den - there were 120 of them - not lions but accusers of Daniel. It doesn't say how many lions there were.
You can read about it in the book of Daniel, Ch. 6