How long does it take for hair to grow longer? (its at my shoulders now)?

You don't say how much longer. I mean, it grew longer today. 

I think the average rate of growth is 0.5" a month. But that is an average and everyone is different. It takes as long as it takes, basically.

Hair grows on average about 1/2" a month so it will take 8 months to grow back to its original length

Oh darling. 

You need to put some hair removal cream in your mum's shampoo.

I've never forgiven my mum for having my hair cut short and when I say short I mean the Beatles had longer hair than I had. 

Get a multivitamin or vitamins aimed at hair, skin and nails.

your hair is growing all the time. it never stops.

Hair grows at about the rate of half an inch PER MONTH.  If you lost four inches, it will take most of the year to get that back.