US revenue (Primarily taxes) for FY2014 was $3.02 Trillion, while defense spending was ~$640 Billion, so you could guesstimate roughly 20% of tax dollars for defense. That ~$640 Billion is out of a ~$1.1 Trillion discretionary spending by the US, while non discretionary (mandatory, things like social security and medicare) spending is over $2.4 Trillion.

If you add it all up, military spending makes up about 16% of total government spending.