Dyeing my hair from plain brown to dark brown?

Hi, I got plain brown hair, not light, not dark, just plain brown. I really want to dye my hair dark brown, just to try it. But, then I'm wondering if I dye my hair with a semi-permanent dye(that lasts for about 1-2 months) into dark brown and then don't re-dye it when my roots start to appear..will that look really weird? and how long will it take until the rest of my hair get the same color as the roots? I'm really clueless about this, sorry!>.<"