The Length of the Exorcism?
In the original Exorcist, where a priest and maybe a Cardinal were talking about who should perform the rites, didn't they say that the first exorcism Merrin performed took a long time...30 days comes to mind, or maybe even longer...and that it "damn near killed him"?
Not that I think watching 3 months or whatever of an exorcism would be terribly compelling...