How long does Splat Lusty Lavender hair color last?

I want to dye my hair purple for church camp (our team color is always purple) but I can't still have an unnatural looking hair color when school starts. So I need to know how long the color lasts before I can use it. I have the Splat brand color lusty lavender, it says it's semi-permanent... but how permanant is semi- permanent?