I plan on bleaching my hair soon. I'm just interested to know what everyone finds more effective -

Bleaching using a 20/30vol bleach twice...

or using a 40vol bleach once.

Or even using a 40vol bleach twice, but only for 30mins each time.....

Sorry, I hope this makes sense!

40 vol bleach is really harsh. I don't know if I would use it twice.

20 vol. here.

40 vol. is for off the scalp bleachings, which is why there are so many horror stories of people thinking they're allergic to bleach. its just too harsh for the skin.

My friend who bleaches my hair (she's a professional) uses 40 volume bleach to do my roots, says my hair won't want to bleach much otherwise (I have brown hair normally). Never had any problems with my scalp, though, and the hair is in pretty ok condition. I guess it depends on the hair/scalp and the person who does it.O_o

If I were to do my hair and my roots (or someone else's for that matter) on my own, though, I wouldn't use 40 volume bleach, I'd be afraid of burning my head and hair, since that stuff is strong. I'd probably use 30 on natural hair and to lift existing colour probably a 20.

However it does all depend on how easy your hair is to bleach, your starting colour etc.

I used 40 volume once and it was awful.
It burned and turned my skin red.
I'd stick to 30 volume.

I used to use 30 volume (my hair is almost black). 1 bleaching was enough if th regrowth was really short but if it was longer I'd do one 60 minute bleaching with 30 and then another for about 20 minutes on the parts that were still golden. Using 40 was too much for my hair and caused a lot of breakage so 2 x 20 volume is probably your best bet. As well, it gives you the opportunity to do any touch ups of bits you missed when you do the second application.
Is this your first time bleaching? If so do a strand test to see how resistant your hair is. Sometimes even dark hair can lift really quickly, especially if it's fine-textured.

whatever u decide, i think gradual bleachings are good with going from very dark to very light....just so you're not beating up your hair in one go. spacing it out a bit gives your hair a break.