I did this in the past and got to a light blonde with only two bleach sessions , (that was what I was trying to get though and I had dyed black hair) you'd probably have to do a third bleach session for silver but you'll have to take breaks in between !! I waited two weeks in between each dye , but I used a color remover first which got my hair to an orangy light brown color so I'd recommend a color remover like color fix that you can purchase online or at a Sally's , ONLY use it on the parts you want to ombré and before you bleach DO A TEST STRAND I can't stress this enough so you know how light you can go without breaking your hair and how long it will take do lots of coconut oil and deep conditioning treatments in between the two weeks till you bleach again hope this helped!

(The picture below is my before and after, no filters only flash)

You don't need to bleach all your hair, just the bits that you want to be silver. Get it done professionally, diy ombres can look really messy and bad

A lot. Go to a hair salon that uses Olaplex, which is gods gift to bleaching.

Dark brown would need to be bleached until white. That would most likely cause the hair to be so burned and weakened that it would break off and fall out. You'd end up bald.