According to an Australian study, 10 minute naps are the most beneficial.
The study compared 5, 10, 20 and 30 minute naps. The benefits explored included sleep latency (how long it takes to go from awake to asleep), subjective sleepiness, fatigue, vigor, and cognitive performance.
5 minute naps - had no measurable benefits.
10 minute naps - showed the most benefit among all measured aspects of the study. Some of the benefits lasted for as long as 155 minutes.
20 minute naps - increased benefit among all measured aspects of the study, but it took 35 minutes in some cases for the benefits to take effect and they didn't last as long (125 min).
30 minute naps - actually impaired alertness and performance immediately after napping. Then, the benefits kicked in and lasted as long as the 10 minute ones.
Incidentally, The City University of New York study found that napping for longer than 45 minutes was detrimental to performance on all levels. Those who nap longer than 90 minutes regularly should be checked for sleep disorders