However long suits the story you are telling.
A slightly more helpful answer would be that it depends on exactly what is being summoned and who is doing it. A powerful sorcerer might summon a minor demon in seconds while his apprentice could just about do it in an hour's ritual after a day's preparation. For really powerful entities you could have a team of magicians working together for seasons or even years to summon and control them. The two year seige of Whitewall may well be an attempt by the Lunars to summon and bind Orlanth.
-- Donald Oddy
Powered by hypermail