According to John Bingham's Running for Mortals, it takes an average of 21 days for an activity to become a habit. Not 21 days of doing an activity, but actually performing the activity on a regular basis over roughly a 3-week period. If you interrupt that cycle, then it's back to the beginning again.
I believe this is why training plans are so important. I have a hard time just lacing up the running shoes and heading out on runs on a haphazard schedule. It's easier for me to commit to a 10 or 12-week plan with pre-set mileage. I'm less likely to drop or postpone runs if I have a calendar telling me when and how long I have to run. It's predictable. It makes fitting runs into my busy schedule much easier, as well.
On that note, I've committed to running three times a week for the month of August. As I mentioned the other day, I've selected a 5K training schedule from Nike+. I modified it slightly to meet my needs, as there was a bit too much of a mileage jump in the first few weeks. It's nice feature of the Nike+ website because you can tailor it to meet your own individual needs, plus you can see your progress in the program on the site.
On deck: a 6K run tonight.