Imperial Sanctum

When do I get there? Also, when do I get to House zu Helter.

Imperial Sanctum is the last mission in Cantha.

You gain access to both House zu Heltzer and Cavalon after completing the Sunjiang District mission(7th one in Cantha).

And when do I get to there?

Err, you'll get there after finishing the 11 missions that preceeds it. (11, because you don't have to go down both the Kurzick and Luxon sides of the story.)

Unlike Prophecies...Cantha is full of locked gates, ect that force you to go through each mission/primary quest.

Go to guildwiki.org, type in a random Factions mission, and at the bottom of the screen you will see 13 missions in order of appearance ingame, unless you come from a other campaign that is