Quick bleaching query - strength vs time

Part of it probably depends upon how quickly you can work, and if you are confident to apply methodically and let it start developing, then I'd say go for stronger.  That said, 'strong' peroxide for roots would probably be 20-volume maximum, as heat from the scalp helps the bleach develop faster than it would on the lengths of your hair.

I'm using 20-volume at the moment for roots, sometimes just a bit diluted if the mixture is too thick.  I don't work particularly quickly, perhaps 20 minutes application time, and another 20-25 minutes of developing, and that seems to do the trick!

Lower volume is always less damaging.

Basically, the higher the volume, the higher the cuticle lifts and the more intense the chemical reaction.

So twenty volume will lift the cuticle slightly and work slowly, but because it's a more gentle process, it's a less damaging one.  Thirty volume lifts the cuticle higher and works faster, and forty volume basically rips the cuticle upright and tears in to the hair.

The closest analogy I can think of is different types of soap.  Liquid or foam hand soaps are usually much gentler on your skin, so even if your hands are in them for a long time, they tend not to dry out too badly.  Bar soaps suck out more moisture, and the skin gets irritated more quickly.  Dish soaps can destroy the delicate skin on the hands if they are submerged in it for extended periods.

The main thing to be concerned with when using lower volumes for longer periods is to wash it off once it's stopped lifting.  All peroxide continues to damage even after it doesn't lift any more, so if 20 vol only takes you to a medium yellow shade, leaving it on longer won't make it paler, but it will make it more damaged than necessary.

Thanks! I shall dilute the peroxide then.