I believe it is set here. Copied from the Balrog. I added Sauron for testing puposes and he never goes away no matter the timer set. Unless I'm setting this in the wrong place I belkieve it's here.
Code:
SpecialPower SpellBookSauronAlly
Enum = SPECIAL_SPELL_BOOK_ELVEN_ALLIES
ReloadTime = 360000 ; in milliseconds
PublicTimer = No
RadiusCursorRadius = 75.0 ; Match the radius in FXList SummonBalrogStart
RequiredScience = SCIENCE_SauronAlly
ViewObjectDuration = 5000
ViewObjectRange = 100
End