I'm assuming you are specifically referring to the "Nintendo Direct", and not "Wii U Direct", "Nintendo 3DS Direct" or any game-specific episodes?
Well... let's have a look...
October 21, 2011 (Japan and US only)
December 27, 2011 (Japan only)
February 22, 2012 (Japan, US, and PAL)
April 14, 2012 (South Korea only)
April 21, 2012 (Japan and PAL only)
June 21, 2012 (Japan, US and PAL only)
August 29, 2012 (Japan only)
October 4, 2012 (PAL only)
October 25, 2012 (Japan and US only)
October 31, 2012 (South Korea only)
December 5, 2012 (Japan, US and PAL only)
February 14, 2013 (US only)
April 17, 2013 (US only)
May 17, 2013 (US and PAL only)
August 7, 2013 (Japan, US and PAL only)
So, the point is... there has always been one every 2 months (although occasionally shown on different dates in different regions. The only exception being the one held in May (which would have been the June 2013 Nintendo Direct, but they had it in May because of the E3 thingy.
So, anyway.... with this in mind, the next Nintendo Direct that isn't focused on a specific game or system, SHOULD be held no earlier than October. However, whether there's an additional "Nintendo 3DS Direct" or "Wii U Direct" or "[insert game title] Direct", or even a "Nintendo Direct Mini", this month, no one knows, but it's a possibility. But the general "Nintendo Direct" will unlikely be until October. But it's September 21 (in the UK), so October isn't that far away.