Mine are the pro gaiter jobbies, and they have been fine up till lately, but now the one zip drops.
Having said that I know a few people who it happened to much more quickly.
Agree with MMax, would have a go at sending them back, I have always found ariat very good at replacing stuff if its knackered within a reasonable amount of time.
I had a pair that did this, actually one was worse than the other, I contacted Ariat, you can get the details on their web site, and they said for me to send them back, about a week later I got a brand new pair in the post!! They were very very helpful and said that this should not happen. Just contact them or take them back to the shop you bought them from and they will sort it out. I suppose you get what you pay for, Ariat are expensive, shouldn't go wrong but also have fantastic customer service if something is not up to standard!!!
There was a post about this on here a little while ago, where quite a few folk were having this problem, with various different styles of Ariat half chaps. I decided to take the plunge and send my fally-down zip ones back back even though they were just over a year old, and they sent me a replacement pair practically by return of post. I was very impressed and recommend that you return them to Ariat