The session time defines the formal temporal bounds of the practice period–i.e. how long it is.
Each practice period must have a defined length. In general we recommend doing at least a minute of practice time and to consider that because, "with social meditation, ten minutes of practice equals ten minutes of time on task" a relatively small dose of practice–we generally recommend 10-to-20 minute sessions–can be a quite potent dose of practice. Because of this, we don't recommend doing more than 30 minutes of social meditation practice at a time, without having time to reflect and debrief on how things went afterwards. For 30 minutes of practice you'll want at least 10 minutes of reflection time after.