Depending on where you live, dosa batter may need atleast 8 to 14 hours to ferment.
If you live in a warm place, it may be easier to ferment the batter in less time.
In winter and if you live in a cold place, you can ferment the batter either in the oven or in the instant pot.
To ferment the batter in the oven, turn on the oven light and place the batter and let it ferment overnight. Please note that we are not turning ON the oven, just the light.
To ferment the dosa batter in the instant pot, transfer the batter to the instant pot. Close the pot with a large plate or a glass lid. Do not use the regular instant pot lid.
Turn the instant pot on and press the yogurt button and adjust the time to 10 hours. If you live in a much colder place, you can ferment the batter for couple more hours.
The next day the batter will be fermented, frothy and will double in volume.