Demi permanent hair dye?

I am currently a blonde-Ish brown colour that I hate (it's light on top and a light ash brown underneath) so I've been wanting to dye my hair a nice light, chocolatey brown. The problem is, I'm still living under my parent's roof and my dad won't let me permanently dye it, so I'm looking into Demi-permanent. How long will it last on my hair? And what is the process of it washing out? Does it just get lighter and lighter until it goes back to blonde? Or will it just all wash out when the time comes? And also, is it damaging if I dye it back to brown whenever it fades out of my hair? Should I dye it out of the box or play it safe and go to a salon? My hair is very short, I'm trying to grow it out of a pixie cut, so it's just passed my ears at the moment