bleaching long hair

Firstly, are you certain all the old colour is out? Bearing in mind that hair grows roughly 6 inches a year, the ends on very long hair are typically over 4 years old. So unless you have not coloured in the last 4 years the colour will still be there even if not visible.
I use cling film or a bag to cover the ends/lengths as it is easier to check how it is lifting. It is necessary to cover to keep lengths warm and moist.
Heat speeds the process up, so can be safely used with lower vol peroxide. it can be extremely risky if working with high vols.
If you are doing the application on yourself, i would definitely not attempt a whole head of long hair in one sitting. Divide into sections, and fully bleach each section and rinse before moving on to the next.

I used to use foil, even with 30vol, but soon changed to cling film as i found that the the plastic actually helped keep the heat in better. My bleach was more likely to dry out when I used the foil as it doesn't form as much of a seal around the hair.