You can really dig down deep into the details when planning a workout, including timing how long each rep should last. But sometimes going with your intuition is a better approach. Consider the findings of a study published in the Journal of Strength and Conditioning Research.
Twelve men with weight training experience did a pair of high-intensity workouts. One involved 3 sets at a self-determined pace. The other 3 sets were timed as 2-seconds for the concentric and 2-seconds as the eccentric part of each rep. Self-determined rep speed resulted in greater volume and muscle activation.