If I have been wearing them regularly (every day) I change them as directed. If I skip a few days, sometimes I wait a bit longer as long as the contacts are still fresh and round. Sometimes they may look fine, but once you put them in your eyes they are very uncomfortable.
To illustrate, you must change your contacts as directed to prevent any problems from arising from overuse age of the contacts. Often, the contacts are overused enough that they will disappear into your eye sack.