How often must hair be dyed and do they worsen damaged hair?

I'm interested in dying my hair - a few blue streaks along the length of my bangs. I've never dyed my hair before, and have been trying to look up some info, but have only ended up a little confused. I'm not too sure on how long blue dye lasts, and based on some of the answers i read for questions here on yahoo!answers, "level 3" or "permanent" seems harmful for your hair. I would like for the dye to last as long as possible, and because I'm not dying my whole head, I don't know whether I need to worry about my roots showing or anything. I don't want to damage my hair more than it already is (lots of split-ends), and I've read and heard of alot of nasty things concerning the effects of hair dye on the health of your hair as you use it continually over time. I'm most likely going to get my hair dyed at a salon, but I don't really trust the stylists and the products they may use so I may do it myself at home. I've also read of dyes with vegetable or fruit pigments and while they may not last long, are they really better? I would really like some advice and words of experience from people who dyed their hair before; thank you in advance if you answer my questions. :)