Rebuild your index according to how much the are fragmented. You can do this by using the new dynamic management views to determine what level they are fragmented. One you know about how long it takes for them to become fragmented, schedule the plan to run then.

You should note that your fragmentation will depend on the fill factor you have specified for the database. If you have a 90% fill factore then you are leaving 10% available for indexes to grow. Once the fill factor "fills up" pages start to split and performance degrades. On another note: if this is a OLTP database and there are lots of deletes, you can have "internal fragmentation", which causes empty space in pages. While this form of fragmentation is less degrading to performance, it should still be addressed frequently to keep performance optimal.

Like I said, indexing is dependant on your environment.