One way to set the length of your sessions is by paying attention
to how you're feeling moment-to-moment. Not many people will need to use this approach.
For some people, it may not work out to do a standard 30 or
minute session. These people may find that, although their sessions begin well enough, there can be a 'shift' during
the session, so that a positive feeling is replaced by a negative one. Usually, the after-effects are positive,
and the unpleasant sensation may be very subtle.
If this happens in your sessions, then you should stop the session
as soon as the positive sensation stops, and note how long the session lasted. After that, always stop the session
at that point. Use a timer. What will have happened is that activity from the target structure will have spilled
over to adjacent structures. The specific sensations will depend on which structures are involved. The length of
session you end up with using this method will depend on the thresholds for the structures in question.
The length of session you end up with will only be valid for
the session design you're using. You may not need to adjust the length for other session designs.