3%

I found this on Sipsey Street Irregulars. As near as I can tell, the numbers are all conjecture, but they do sound reasonable.

During the American Revolution, the active forces in the field against the King's tyranny never amounted to more than 3% of the colonists. They were in turn actively supported by perhaps 10% of the population. In addition to these revolutionaries were perhaps another 20% who favored their cause but did little or nothing to support it. Another one-third of the population sided with the King (by the end of the war there were actually more Americans fighting FOR the King than there were in the field against him) and the final third took no side, blew with the wind and took what came.

You can read the whole thing here.