I give up on a trend when I've become tired of it. In this category, you can place ruffles on blouses. I loved the look when it first came out but boy have I OD'd on it. Also in this category is the color gray. That should be a classic color, it looks great on me, but I am tired of it because I now own way too many gray tops! It annoyed me immensely last year because when an item was offered in petite, the choices would be black, white and... gray. That's it. But I digress...
I also wonder about this question because for example I've always viewed knee-high boots as a trend. They *used to* come in and out of fashion... but now it seems they are here to stay. I am all for that, because knee-high boots are so freakin' practical in Boston weather. Likewise, I hate to see skinny jeans go out of style because they are perfect for tucking into the boots.
Wedge heels are another one: it seemed to me that wedges would come in and out of style, but they've lasted for awhile now. Which is great, because walking on cobblestones is a lot easier in a stable wedge.
Then you have trends like florescents. My guess is that will not last all that long. Perhaps color trends is another category altogether? Colors come in and out for a season. I snap up colors that work for me when they are available, and don't worry too much about trendiness. A color that flatters will always be on trend, far as I'm concerned.