Firstly, rest assured you are nowhere near totally blind. The teacher I learned from healed through -12 and -14!
If you really wanted to have the quickest change possible - if you could stop using glasses, computer, and tv you would certainly get the quickest changes. If you pay attention you will see the first result within the first day of that change.
I recommend palming. If you could palm for hours in a day you would see HUGE changes not only in your vision but tension in your shoulders and neck and back would greatly dissolve.
However, with regards to how long per day to exercise - the basic point of Bates method is that stress and strain are the causes and if you can live without stressing and straining you will have your perfect vision. So it's not that you have to be doing any exercises, but for many people it takes some body usage awareness and relaxation activation in order to dissolve and resolve all the tensions that have been carried for so long.
For many people events happened that resulted in a stress and strain and since those events were not resolved in time, glasses were prescribed at which point the stress and strain began to be ignored out of sight. I am not suggesting you begin searching for events that may have caused a strain or stress like that, but wish to inform you that this is a bit of self-discovery you are embarking on and releasing your tension may also be releasing a piece of history to be integrated into your dynamic growing presence.
I'd like to add that if you do remove internet and tv and so forth from your life, make sure you have something you love to do instead. If you designate your time to your health you will certainly be greatly rewarded even if you get bored or find it difficult to accept your conditions.
I also recommend purchasing "pinhole" glasses they will be very helpful to you.
If you are interested in a free 25 minute consultation (650)265-1776 or add me on skype david.shlomo.hestrin