How long this takes all depends on your original hair colour and hair strength. I rather waited a month in between bleaching to give my hair enough time to recover.
Before putting any chemicals in your hair, test the elasticity. Wet your hair, take a single strand and see how far it stretches before it breaks. Hair should not stretch very far. Damaged hair can stretch up to double the length of the hair without breaking.
If anything in the How-To is unclear, just ask and I'll try to clarify it =]
This is basically what I started with. Faded purple hair.
Buy Kair's Blonde Highlighting Kit or something similar, NOT bleach. This is probably my biggest trick in dyeing hair. Bleach leaves hair very yellow/orange, whereas a blonde highlighting kit will take away most of the yellow.
How many boxes you need depends on how long and thick your hair is. I use 2 boxes for my whole head.
Mix the highlighting kit as instructed and add it to all of your hair. Leave it in for the longest time allowed. For mine it was 55 minutes.
Wash it out after the time and dry your hair. My hair came out like this the first time: very light blonde but still a bit dark to use the platinum blonde shampoo.
Use a hair treatment to help strengthen your hair and leave it to recover before lightening it the second time.
LEAVE YOUR HAIR ALONE UNTIL THE ELASTICITY IS NORMAL! Otherwise your hair will fall out if you add chemicals to it again. This could take a week, or a couple of months, depending on your hair, but I left mine for a month just in case, using damage repair treatments daily for the entire time.
When your hair is ready, lighten it again.
If your roots have grown out and look bad like mine did, lighten them in the same way. Add the highlighting kit to just your roots. I only had to leave it in for 15 minutes over my regrowth to achieve the same colour as on the rest of my hair.
Keep repeating this until your hair is really white and cannot be lightened further.
Then buy platinum blonde shampoo. This is usually only found at hair salons. I used Paul Mitchell's Platinum Blonde Shampoo.
Follow the instructions of the shampoo, but leave it in for a few minutes longer for a whiter blonde.
My shampoo says 2-5 minutes, but I left it in for around 10 minutes once a day for a week.
Keep doing this until you achieve the colour you want.
Once you get the colour you want, only use the shampoo once a week to lighten the colour a bit again. Wash your hair with shampoo for colour treated hair and enjoy your awesomely light blonde / white hair! =]