How much to heat mustard oil before cooking? We use mustard oil.

How do I know if the oil is ready for cooking?

My mother always waits till the pungent smell goes off. 
I can't put my finger to know if the oil is ready.
To check if mustard oil is ready to use, add onion peel: if it browns immediately, the oil is ready to use.
You can also notice the change in fluidity of the oil.