Anything more than 10 minutes between vehicles during peak time is absolutely unacceptable for a transit system, no matter how low the ridership is. From (at the very least) 8am-6pm you should be able to walk out at any time and catch a bus/train without planning
And I say “no matter how low the ridership is” because if it’s longer than 15 minutes your ridership is going to stay low because it’s easier to drive.