In truth, we don't. A large group of tree frogs lays eggs in our winter pool cover almost every April, and I have to take them from the 'pond' so we can open the pool. They spread disease. I lost about 50 tadpoles that way. Get rid of the mosquitoes.
Tadpoles really vary in terms of size, and lots of it depends on temperature.
I have had froglets in one tank that is often in the sun, and baby tadpoles less than a half an inch long in another tank in the shade. They were siblings. Tadpoles generally grow faster in warm water, but be very careful, because in the water is too hot, they will die. They don't like having many other tadpoles in the tanks. I have tanks about one foot wide and a foot and a half tall, and I only keep 14 tadpoles in each.