My cornrows last 2-3 weeks at most (they go that long if I mist them with an oil spray and wrap at night, if not it would be a few days less) and my individual braids, 1-2 months usually. If you are going to do it continuously, I would be careful of them being pulled too tightly as that can cause the hairline breakage others described, and I would also vary the size and pattern of the rows each time so the same areas aren't being stressed. I do my own to avoid that.

Sisa, it's great to finally see your face! I so love your long hair!!!!! That must have taken a while to braid!
Get used to me. Black, confident, cocky; my name, not yours; my religion, not yours; my goals, my own; get used to me. -Muhammad Ali