In arid trees and shrubs we found that 0.5mg/Litre is the best to induce maximum rooting.

Do you want to treat Arabidopsis seedlings for 10 days with NAA, or you want to use 10 days old seedlings to treat with NAA? For lateral root induction, a concentration between 1-10 uM is often used. If you want to test something new, I would anyway test different concentrations between 0.01 and 10 uM. If you plan to do a long treatment and if you want to avoid unnatural root phenotypes, I would rather focus on the low concentrations.

I think for your situation you may use 20 to 50 ppm solution.

I'd suggest to use different concentrations. If you still want the roots to grow considerably, go for a range of 15nM-100nM. In our hands, everything higher than 100 nM NAA strongly inhibits primary root growth. 

Thanks Dr. Hans Motte. I want to treat 10 days old seedlings with NAA for short duration like 24 to 48 hrs in the liquid media and then transfer to normal media for inoculation. 

You may use various range from 10 to 100 ppm solution