On a regular basis, the question is asked when a table should be defragmented.
Often, several different people respond with quite different answers. There is a good rule of thumb (if at least 100 pages are fragmented), and I am not proposing any other "rule".
But people hardly ever realize how big the impact of fragmentation can be. And most inexperienced developers don't know what contributes to fragmentation.
In the article
Fragmentation Hurts Performance, I have tried to explain the different types of fragmentation that can occur, with some practical number of their impact, and 13 tips to prevent, reduce and remove fragmentation.