As a rule of thumb, always check out the Monarch Watch website for information on everything:
When do you tag Monarchs?
As the length of daylight shortens in mid August and September, monarchs in northern latitudes, i.e. near the Canadian border, begin to migrate. Monarchs farther south will begin their journey a few weeks later. Tagging and monitoring should begin in late August in all regions, with a concentrated effort made in September and early October. A GOOD RULE: when the wild asters, especially A. novae-angliae, goldenrod and Joe Pye weed are in bloom, the monarchs are migrating. In much of the lower midwest, migrating monarchs are attracted in large numbers to a tall late blooming thistle (Cirsium altissimum) several species of sunflowers and other species of Asteraceae.
Additionally, you can determine the estimated peak of the migration in your area based on latitude:
Peak Migration Dates