According to Apple the Lightning cable can extend up to 2 meters (6.5 feet) and work optimally. You can likely find 3rd party (non-Apple) cables that are longer, however Apple onnly recommends and sells cables up to 2 meters.
Lightning is a proprietary computer bus and power connector created and designed by Apple Inc. and introduced on September 12, 2012 (2012-09-12), to replace its predecessor, the 30-pin dock connector. The Lightning connector is used to connect Apple mobile devices like iPhones, iPads, and iPods to host computers, external monitors, cameras, USB battery chargers, and other peripherals. Using 8 pins instead of 30, Lightning is much smaller than its predecessor, which was integrated with devices like the iPhone 4 and the iPad 2. The Lightning plug is symmetrical (same pins on either side), so it can be inserted into a Lightning receptacle in either orientation. The plug is indented on each side to match up with corresponding points inside the receptacle to retain the connection.