Answer Description
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.
Wikipedia
Lightning is a proprietary computer bus and power connector created by Apple Inc. 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 significantly more compact than the 30-pin dock connector and can be inserted with either side facing …
Lightning (connector) - Wikipedia, the free encyclopedia