How long do real hair clip-in hair extensions last?

How long do clip-in hair extensions usually last? I got real remy hair extensions about 4 months ago, and I feel like I need new ones. They are a bit dry, a little brittle, some of the hair is coming out of the wefts or breaks off, and one of the wefts is pretty much ruined. They aren't in terrible conditions, I can still wear them, but I was just wondering if I need new ones already and how long hair extensions usually last?