The physical manifestation I have experienced only lasted for about 8 minutes. Towards the end the temple psychic reported that the presence of the goetic entity was still as strong as ever but to the naked eye the demon was blurring and becoming indistinct. I instructed the entity to show itself clearly as it had originaly done and was now bound to obey me in my wishes. As far as we could tell it was genuinely trying but would 'lapse' and blur again after making the effort at point of sword. Most business with it was completed so license to depart was given.
As far as I can tell the problem was dittany of crete lessening output. I'm thinking that having a censer in the triangle as well as a battery of them in the circle and gently fanning the smoke towards the triangle will maintain sufficient smoke particles/accumulator for a longer manifestation.
Any ideas on how to maintain steady smoke for longer would be greatly appreciated.
Or perhaps I'm missing something here, if so could those with more experience point me in the right direction.