How long should I leave my bb in for?

An hour is the maximum. You need to keep an eye on what's happening. Keep checking by scraping a small amount off a small section of hair and see what colour it is. When it looks right wash it off. But don't exceed the hour.

Thanks! 🙂
Edit: does this include the time it takes to put the bb in? I have quite long hair and applying dye usually takes a while,I assume it'd be the same for applying bleach.

Technically you should include that time. I often don't, but my hair is normally the right colour long before the hour is anywhere near up.

I'm going to go and read the tutorial too - but what is 'the right colour' or is that different for everyone depending on what you want?

Yeah, it's the result you want.

Basically the right colour depends on what colour you want to achieve in the end, are you going blonde or are you creating a pre-lightened base for a colour?

Soooo I attempted the bb today. It went alright and I'm pleased with how much colour came out of my hair (it used to be purple); except for my roots. I think I may have rinsed them out too quickly and now they're quite bright, almost orangey. I want to dye it Alpine Green and a strand test showed that it does take on the roots, but I'm scared it might turn out as a different colour on the roots than on the lengths. And then I'd end up with a big line. So maybe I should wait a few days and do another bb just for my roots? What do you guys think?
Btw: I love the colour my hair has gone! Just too bad the roots really don't go with them 🙁

I don't think it'll be a problem, though it might be as it fades.

Just so you know, you shouldn't bleach bath fashion colours unless it's a last resort. You can end up with some very unexpected results.

Oh right, I thought a bleach bath was fine to apply on fashion colours also. What would you suggest to do instead then, to get the colour out? By the way, my hair was already faded as much as possible (I've been washing it with head & shoulders for a few weeks and it's been a few months since I last dyed it) 🙂

There's a lot of ways. http://www.hairdyeforum.com/index.php/topic,1704.0.html

There's also colour removers. They don't always work well on fashion colours, but you should always try them and the methods from the link above first. Honestly, using head and shoulders is ok, but it doesn't remove that much.

For example though, if you bleach bath a purple it frequently turns out green.

I've been using Alpine green on a portion of mine, over my virgin roots (dark ash blonde) and dyed reddish brown lengths and it is very forgiving of the base colours. I will warn you though, in my experience it is VERY long lasting, I've not applied any in about 3 months and I've been using head and shoulders and other harsh shampoos and the portion applied over a permanent dye has not really faded at all!

Thanks lauralei, that's good to know! I've not dyed mine yet as I really like the minty green colour I ended up with. I'm just hiding my roots with a hat for now :p

If you like the minty green and want to keep it you could add a few drops of green dye to some conditioner to make a pastel green dye for your roots!

I would suggest manic panic atomic turquoise for mint! It's a lovely mint when diluted 🙂