How long should each session in a meditation course be?

Long enough to settle, which for most people means 5 to 15 minutes when starting out and 15 to 30 minutes once a practice is established. Shorter sessions done consistently do more than longer ones done occasionally. In StillMind the length is worked out for you from your experience level rather than left as another decision: beginners get around 5 to 15 minutes, intermediate 10 to 20, and advanced 15 to 30, with each session in the course given its own length based on what it is teaching.