There is no specific factor or determinant as to how long a frog can stay underwater. The Tadpoles live exclusively in water till it becomes fully matured. A frog or a toad can remain submerged as long as it wants. The amount of oxygen present in the water could also be a factor. Your frogs will not stay for long if the water is short of oxygen, that is because they need a large amount of O2 to breathe well underwater.
Water temperature can also be a factor for determining how long a frog would remain underwater. Frogs prefer between mild to cold temperatures and wouldn’t hang on for long in water whose temperature is beyond normal. That is because they have soft skin, and the high temperature of the water can easily damage it.
A biological need for food can make it come out of its aquatic environment faster than it normally would. Frogs need food to ensure their survival; all animals, including humans, need food to survive. They would come out of the water if they feel hungry, they feed mostly on insects. So they have to go out in search of food.
The presence of a predator can also make a frog come out of water earlier than expected. Many other factors, such as the species, aquatic activities, and some environmental changes can also affect how long a frog can stay submerged.
However, there is no scientifically proven answer as to how long a frog can stay submerged. As long as the conditions are favorable, a frog can wait indefinitely in water.