Ruth, I'm looking back at the schedule my coach pencilled in and he had me doing bricks at least once a week starting five weeks out.

Week 5: Maintain reg. training schedule w/ 1 brick (short run)
Week 4: Maintain reg. training schedule w/ 2 bricks (short runs)
Week 3: Maintain reg. training schedule w/ 2 bricks (longer runs)
Week 2: Run focus, w/2 bricks (longer runs)
Week 1: Race Simulation, then taper week (3 VERY easy, short bricks)
Week 0: RACE DAY

I didn't follow it exactly (work got in the way) but I did what I could. Hope this helps. Good luck!