Well, honestly, not very.

Here's a quote from General Washington:

"To place any dependence upon militia, is, assuredly, resting upon a broken staff. Men just dragged from the tender scenes of domestic life - unaccustomed to the din of arms - totally unacquainted with every kind of military skill, which being followed by a want of confidence in themselves when opposed to troops regularly trained, disciplined, and appointed, superior in knowledge, and superior in arms, makes them timid and ready to fly from their own shadows."

He also blamed the militia for a lot of problems within the Army, and didn't really have many nice things to say about them, which you can see in this letter: http://www.shsu.edu/~his_ncp/Militia1.html

Basically, the militia generally wasn't trained enough and wasn't effective enough as a military force to fight the war independently. That said, I don't think the militia were as a whole useless- a good portion of the Continental force at Cowpens was militia, and they did an admirable job fighting and winning against the British.

Really, with the militia, it's hard to say as a whole. Some militias were horrid, and others were hardened soldiers led by experienced partisans.