Cornell AppDev's course-grab scrapes the class roster every 20 minutes to get the latest availability of courses (source). 20 minutes is pretty long considering how quickly courses fill up.

Cornell CIT exposes an API to get the most recent data in real-time.

This a toy implemenation I spun up to track two courses i'm monitoring. In summary, it polls the API once every minute to see if the course is now open and sends me an email if so. My math shows this to be a 95% improvement in reaction time.