I also wouldn't assume that any game will last more than a session at level 30. Fight Orcus/Demogorgon/Tiamat/whatever. Wrap Campaign.
Heh, so either way you won't do more than a usual 10 or so encounters at that level. Even if it takes forever to do the encounters![]()
That's the final level. The summit.
There's no way I would spend a single session on it. I might even have it be more than the standard 10 or so encounters (but many of those would not be long drawn out Epic fights, just minor obstacles towards the main goals), just to make it truly Epic.
But a single session? How anti-climatic.

Yes, most of the game is about killing foes, acquiring loot, and acquiring new shiny abilities, but level 30 is about wrapping up the Epic Destinies. Making the world safe for Democracy, err, Anarcy, err, Monarchy, err, whatever.

Fighting Orcus/Demogorgon/Tiamat/whatever is/are the subplot(s) at that level that add to the real plot and overall campaign goals.
But, there is no way I would just wrap up the campaign with a single session massive combat against a Deity. That's just another fight. I'd wrap it up with Earthquakes and Kingdoms falling and Typhons and the new order rising after the old (abet, many worthwhile older elements would remain if good PCs were successful in routing evil or whatever), and definitely quite a few fights against Deities/Demons in the process.
Depending on the campaign, the world might not be Points of Light anymore. It might be Points of Darkness where the Darkness retreats to strike back another day.