Here's my initial estimate: zero.
I mean, what actual threat to America's security is the military currently fending off? Are there any countries that would invade the United States if the Pentagon's budget were 10 percent smaller than it is--which is roughly what $500 billion in cuts over 10 years would amount to?
The main threat to national security you hear about is terrorism. And, so far as I can tell, a big chunk of the money spent by the military to address that problem has made the problem worse. The invasion and occupation of Iraq provided massive propaganda for terrorist recruiters (and the consequent regime change created a new ally for Iran, which is said to be our nemesis and a backer of terrorists). The war in Afghanistan has also been a Godsend for Jihadist propagandists--while, in the bargain, destabilizing Pakistan and making its nuclear weapons more likely to fall into the hands of extremists.