Two extreme (and unlikely) Fog of War conditions could be:
1. Full on - each time you enter the Cyrodiil campaign instance the map is greyed out (which i hate for areas i've previously explored), there is no list available anywhere as to who is in the instance with you, with the notable exception of your guild mates whom you are in contact with. You only know if an enemy is close by through the sounds of combat etc. Reports from Fortresses you own that are under attack are sketchy and vague and somewhat 'dated' - "Fortress Kvatch sustained an attack several hours ago" or "Fortress Kvatch watchtowers report seeing enemy movement 3 hours ago"
2: None - each time you enter the Cyrodiil campaign instance the map is fully drawn - three faded color areas represent the territory under each factions control - a fourth color indicates neutral/contested territory. Small color keyed dots represent all the interesting information: siege weapons that are built, siege weapons currently under construction, resource locations indicating what they are and under who's control. Other dots represent every individual in the instance and where they are (catastrophic in my opinion), scaled and real level, class, number of kills vs. losses etc - if these dots are blinking they are currently engaged in combat.
Any Fortress in Cyrodiil would give very detailed reports in a timely manner, even if it concerned an encounter between the other two factions - "AD Fortress Kvatch reports troops from DC massing and anticipates an attack", "AD Fortress Kvatch is being attacked by 38 members of the Ebonhart Pact supported by 3 trebuchets and 5 ballistae and is expected to fall within 27 minutes unless they are repelled"
Somewhere between these two extremes lays what will actually be put in place.
I welcome speculation of the various possibilities and combinations as to the amount of information you think should/should not be divulged within the Cyrodiil campaigns. (ZOS has already stated that there will be notifications from Fortresses under attack - i don't believe we know the details yet)
(Since the varying degrees of info available could greatly influence an individuals PvP game play experience, strategy and tactics by either enhancing it, or ruining it, please try and read and understand other individuals posts regardless of which 'side' the poster is coming from, and if you disagree, be it somewhat or totally, to respond in a civil and engaging manner that invites even further and deeper discussion)