If you live in warm place, leave the batter covered in the counter top. The batter will ferment with in 8 hours.
If you live in cold place, in winter you need at least 12 to 14 hours for the batter to ferment. In summer it may take 8 to 10 hours to ferment.
But in winter and if you live in a cold place, place the batter in the oven with the light on.
You can also 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 12 to 14 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 ready. The volume will not double with rice flour dosa, but will ferment very well. It will be frothy and will have bubbles on the top.