The Vanguard: Saga of Heroes open beta will close just before midnight on 23rd January according to Sigil Games Online and publishing partners Koch Media and Sony Online Entertainment.
That's in anticipation of the game's launch on 30th January, of course. Those who have pre-ordered the game as part of its special campaign will be able to gain access early - with more information due soon.
As for what will happen to your beta characters - Sigil says that they will be transferred over to a test server, which will become active at some point after launch, the idea presumably being to help, well, test things.
For more on Vanguard, check out our recent impressions.