It's very important to understand you cannot go from a dark brown
to a platinum blonde in one bleaching (processing).
There are "levels" of  "lift" (lightening) your hair must go through until it reaches maximum lift.
The higher the lift, the lighter (blonder) your hair will become.

In our opinion, it better to use a lower lift over the course of a few days,
than to do a single day bleaching.
Spread it out and you're less likely to fry your hair.

You should find your current hair color level below,
then figure out the number of lifts to get your desired hair color.
You should also understand how many levels your bleaching product and developer will lift.