Bleaching hair (platinum blonde) men long hair

It is definitely possible, but it won't be easy.
You will need to bleach more than once, so you will have to endure some orange tones/brass along the way if you want to do it safely.
To stay Targaryen white blonde you will need to constantly tone, this is not a low maintenance look. Plus you will have dark roots, they will start to show almost instantly, so another thing to bear in mind. You will need to touch them up every 4-6 weeks, but with such a light colour they will be visible.
Also to bleach light enough to tone to white will be damaging to some degree, bleached hair is fragile, so you will need to change your haircare routine.

The safest way to go from dark to light is slowly.
You will need a professional quality bleach with a low volume of peroxide. After the initial bleaching you need to wait at least a month before bleaching again. You may need  to then do a bleach bath to ensure it is all an even pale yellow, the colour of the inside of a banana.
Then you can tone it to platinum with violet dye diluted in conditioner

When Wicked Pixie says your roots will show quickly she isn't kidding, one of the reasons I never stick with blonde or any light colour is you literally start to see black dots at the roots after a day! Not everybody can achieve white blonde either, I've never got past pale yellow no matter what I've tried, but you should be able to get it pretty light if you're willing to be patient and put up with some possible brassiness in between. (Meaning that when lightening dark hair to blonde it will first lighten to ginger then yellow and only then to a light yellow that can be toned to white, so you may need to go through a ginger or golden blond phase on your way to your desired shade).