Big mistake with hair.. bit scared, please advise :(

Permanent colours over bleached hair tend to fade very quickly, so you should see a noticeable change in just a few washes.
Try a hot oil treatment, it will help to put some moisture back into your hair and pull some colour out at the same time.
I like a mix of coconut and olive oil applied warm and covered and left on overnight. In the morning work some conditioner through it, (not shampoo) and rinse.
Coconut oil applied sparingly to dry ends is great for making them less frizzy, smooth and shiny.

Thank you for the great advice, so far I've just been applying a lot of mythic oil (argan) at night and I've also done the mixture of hot olive and coconut oil over night and whilst it feels a little smoother, it hasn't really made the colour any lighter (it's almost black is such a dark brown).

I was wondering about a couple of things and hoped people could advise?

1.) How long would it be until I could lighten the hair to a lighter brown rather than black/brown, and would I need to use bleach for this as I wanted to steer clear of bleaching it for a looong time.

2.) Should I consider getting my hair cut to try and get rid of the damage? It's already very short as mentioned but if it will make it better I'm certainly willing to do it. The hair isn't gummy when wet but it does feel and look dry/straw like, also a lot of strands tend to come out now when washing it.

thanks in advance xx

If you want to avoid bleach, you can use a colour remover to remove the pigment from the box dye.  Scott Cornwall's Decolour remover works well for me (don't get the stripper, that one contains peroxide, you want the remover one).  This can help take out some of the dye molecules, and isn't damaging to your hair in the same way bleach is.  It can be very drying though, so you'll want some good deep conditioning treatments for afterwards.  Then you could put an Adore semi brown over the top to tone it to the shade of brown you want. 
As far as cutting your hair goes, if you feel it's damaged on the ends and needs a good cut, then it might be a good idea, but only you can assess how damaged you feel your hair is.  However, if you do decide to get a trim or a cut, it might be a good idea to do the cut first, before the colour remover, as the remover is more likely to damage/dry out hair that is already damaged.

Mythic oil is made up of more silicones than anything else so don't let it fool you, it'll stop your lovely coconut oil treatments getting into the hair.  Give it a good clarifying wash before you use the coconut oil again, it should work a bit better 🙂