How accurate is the location detection on the iPhone's Find My iPhone app
The generic answer you can find at Apple support website

About location precision or accuracy
Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, Bluetooth, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, your device can determine your location using crowd-sourced Wi-Fi and cell tower locations or iBeacons.

If you want to have specific numbers, in meters, approximately they would be next (according to the 2009 study):

3G iPhone w/ A-GPS ~ 8 meters
3G iPhone w/ WiFi ~ 74 meters
3G iPhone w/ Cellular positioning ~ 600 meters

In 2011 Dr. Zandbergen tested several Android smart phones. Here he found the accuracies to be slightly better than the 2009 study. They ranged from 5-8 meters. It is likely that the iPhone 4S/5 is within this range as well. It can also be assumed that iPads and other Android tablets will be comparable.