I had both my struts replaced under warranty, not long after it's 6 month anniversary :shock: One was leaking quite badly causing the hatch to slam, they did the other, well just because. The replacements are about a year old with no issues yet. IMO struts are just a maintenance item, sometimes they may last a while but eventually they will leak and fail to hold up the hatch. I got tired of replacing them on a Geo Metro I had so I purchased a neat little plastic arm that held the hatch open even though the struts were shot. It worked fine for me but I was always worried someone else would either hit their head on the hatch if they didn't know about the arm or use the arm and then forget to pull it back when closing the hatch, therefor breaking the little plastic arm.
I believe a lot of it has to do with how often they are used, I had struts on the hood of a Buick Lesabre many years ago and they never wore out, of course I didn't open the hood nearly as much as something like a hatch. Very good idea to occasionally check the struts for oil build up, especially if your warranty is about to expire, although I wonder if they might say it's just a maintenance issue if everything worked ok but they just looked dirty.......once they start leaking oil, failure is eminent, how long is hard to tell.