SpaceX has shown this tempo for short bursts, but I think that it will take a little time for them to become comfortable enough with their procedures and processes to maintain a launch cadence of two per month. Having multiple launch pads and multiple launch crews helps.

Launching rockets has typically been stressful for rocket and payload launch crews (meaning long hours of performing time-critical tasks, starting a few days before a launch), and a little decompression time has typically been given to crews between launches (meaning regular hours doing routine, non-urgent tasks). Once launches are made less stressful on the crews, the tempo should be much easier to ramp up to an even higher cadence. Once it is as easy and routine as getting an airliner loaded and underway, crew stress levels should be low enough to maintain a high cadence.

Do not worry too much about customers. SpaceX has quite a manifest, some of which is backlogged (about half way down the page is the list of future missions):
http://www.spacex.com/missions

SteveC: Not quite. SpaceX has launched 10 times this year, in 7 months. They need to launch 14 more times in the remain 5 months to make a rate of 2 a month in 2017.

Ben: This was a barter deal between ESA and NASA. In exchange for providing the launcher for the James Webb Space Telescope as well as one instrument, Europe gets a share of the telescope’s observation time.

The Mercury thingy can basically launch every three month. JWST is 10 times more expensive and the worst principessa ever. Having her waiting on the Devil’s Island, or anywhere near Earth, is completely out of the question! 7 years of bad luck if that mirror breaks. Or even gets a millimeter unhinged.

Please, put that mirror on the black wall,
to see who were the first star of all.

(I fear ESA sabotage here. Theirs own is the BeppiColumbo, but JWST is only paid cargo, with some contract clause to be exercised)