Part of the answer depends on where you'll be working. If you're going to be working in Northern Virginia, then you can look pretty far out in Northern Virginia (Centreville, Manassas, or beyond). If you're going to be working in Maryland, you can look 25-40 miles outside of D.C. on the Maryland side. What you don't want to do is live in Fredericksburg or Manassas and work in Prince George's County. Or live in Columbia and work at Tyson's Corner.
You might also consider having a roommate or two. That can cut down your costs substantially.
Hope that helps.