Get to Mill Road 2 hours prior to kick off and you will be fine. It normally fills up about 1 and a half hours prior to kick off. I always aim for 1pm if 3pm kick off. Blackburn game will be very busy so perhaps get there 2h 15mins prior to game.
Earlier post OP referenced the Racecourse not Mill Road, don't think the Racecourse has the same issues re filling up and is usually fine.