Well depending on the person, and how you go about it, it could last for 30 seconds, 5 minutes or all day. (Never Nair your balls...even in a bet)
If the hair is thick, use the Nair shower version (thick cream with a green and white sponge) If it isnt too bad, you can try the sensitive. I believe my neighbors pops uses a roll on kind.
Anyways just be sure not to scratch your back or rub it or anything, because this will cause little breaks which will cause it to burn more (thats why it says do not use on broken skin) Also any acne on your back will also cause it to burn, since it can work its way between the black heads and burn. (coincidentaly clears up the black heads though
)
Be sure to pat dry, because rubbing will irritate it. You can apply a cool compress if your back is burning. Just run a wash cloth under cool water and apply as necessary.
Yes I know a lot about random stuff lol!