How long does hair take to change from Brown to blond

[Dav7]

I have light brown/ fairish hair and just bought and used a whole box of Just for Men blond hair dye. I basically just rubbed it over the hair about half an hour ago and then washed the hair with shampoo afterwards. However, I can't see any changes in my hair colour, so how long does it take to go from brown hair to blond hair? Also, the only reason I'm doing this is to make MPB stand out less.