Removing hair using Nair doesn't keep it from growing back because eventually your hair will grow back after you use the Nair.
But using Nair doesn't affect the length of time it takes to grow you hair back or affect whether it's thicker or thinner hair that grows back.
Your hair will grow back but will be the same thickness of hair and will grow back at the same rate as it did before.
To keep hair off your legs or other parts of your body with Nair you have to use it again and again when you see hair growing and if you use the Nair before the hair gets too long then it won't be as hard to remove the hair from your body as it would be if you wait too long.
Another option to get rid of unwanted body hair is to have laser hair removal done but that costs more but most times the hair doesn't grow back after laser hair removal although I've known some people who have had their hair come back but it took a few years.