Dosa and Idli batters should be fermented for best results.
Especially Idli batter, Dosa is versatile and is very forgiving! you can make dosa immediately with the batter.
Idli, on the other hand, has to be fermented, I have said earlier the fermentation may take place from 7 to 8 hours to 10 -12 hours! depending on the climatic condition. If living in tropical weather the sun helps quickly in the fermentation process. If you are living like me in a cold country especially in winter you have to look for places where it is very warm like near the heater, boiler room, oven with lights on and sometimes I end up adding a pinch of soda to the batter for it to ferment. Also, you can add a pinch of Eno fruit salt to the batter just before making idlis (after the fermentation process!)