It takes the same amount of time, 5 hours.
The number of bakers is directly proportional to the number of muffins made, and the number of hours is inversely proportional to the number of bakers. This means that if we double the number of bakers, we will also double the number of muffins made in the same amount of time. Conversely, if we halve the number of bakers, we will also halve the number of muffins made in the same amount of time.
Therefore, if we have 100 bakers, they will be able to make 100 muffins in 5 hours, the same amount of time it takes 5 bakers to make 5 muffins.