The free ones are normally limited to once per year.
I wasn't on that short of a time table, but I knew I was buying (building actually) a house. I did a credit check when ground was broken - from that day forward I made sure everything was paid ontime (OK I was a bit sloppy before that) closed a few random accounts that I had opened over the years that I hadn't used in years. By the time 9 months rolled around my score had risen enough to put me in the best lending range - my real estate person was actually shocked. She told me that she had never had someone improve their score during the process, but frequently they had made it worse.
At the time, my pre-ground breaking rate was locked at 6.875%, I saved a full point by improving my score.
Of course that's nothing on the 15 yr 2.875% I have today that I refinanced into a few years ago! Cut seven years off my loan, and actually lowered my mortgage payment by $10/month