For how long will the hair begon after using Nair?

It depends on how fast your hair grows. I use it once a week even though my hair is fine and blond. Sometimes my hair is longer; sometimes it seems shorter, but it does grow back within a few days. It never seems to last for longer than that with me.

did rhia product cause a rash

No. The rash may have been caused due to leaving it on too long or rubbing it in. When using hair removal products, product should be tested on a small area first to see how your skin will react to the product.

Is the hair going to grow fast and getting thicker??

I only used this once as it was too harsh on my skin. I had an adverse reaction to it.

Does your hair grow back thicker? I used the other night. Love the results. Scared it'll grow back thicker!

No, but it does grow back and depending on your rate of growth should be reapplied . Use Vaseline Short time after cleaning it off if your skin. Is sensitive.