When I lived in Birmingham in 2008 the service from Redditch was halfhourly and this combined with 4tph starting / finishing at Longbridge, combining to provide a 6tph frequency from Longbridge to Four Oaks. The Lichfield service was also every halfhour. Of these 6tph, 2tph didn’t call at Duddesdon.
However during leaf fall, the Crosscity only ran at 4tph, toward Birmingham from the south this was an annoying 10/20 Min interval, but the other way round was more even.