Not sure if you've done it yet, but I am a hairdresser and have a lot of experience of colouring/bleaching hair as well as having my own hair bleached for around ten years on and off.
If your hair is dark then you will definitely need to bleach it more than once. It will go red, then orange then lighter orange, then dark yellow then pale yellow. You need to bleach it to pale yellow before any toner or blonde hair dye will work. If it's any darker than pale yellow even a very ash toner or dye won't stop it looking yellow.
You are likely to get some breakage when you bleach it no matter what. How much breakage you get depends on the condition of your hair and what previous chemical treatments it has had on it. The darker it is to start with, the more damage lightening it with bleach will do and if you already have a dark hair dye on your hair it will take considerable bleach applications.
Generally, bleach stops working after about an hour or so because once peroxide (the stuff you mix with the bleach powder) is in contact witht he air it oxidises and turns to water. For the same reason once you open the peroxide unless it is sealed again it will become useless.
It's really difficult to bleach your own hair well. It's likely to go very uneven and yes, the roots will ighten much more quickly than the ends because the heat of your scalp makes the bleach work faster near the scalp. For that reason, unless you can avoid your roots with repeat applications once your roots are light enough, you will get a lot of breakage.
Bleaching hair is really a professional job and once that plenty of professionals aren't even prepared to do.