It currently does not have a duration. It should receive one in the December 15th errata (assuming that doesn't get delayed to January on account of holidays).
The most balanced answer is likely to have it last either until the end of your current turn, start of your next turn, or end of your next turn.
If you absolutely need an 'as written' answer, then it is instantaneous and has no lasting duration, so does absolutely nothing. The next closest to as written is that it lasts _forever_, even months or years later. Both answers are, of course, ludicrous. WotC is well aware about its lack of duration and it will be fixed.
A popular interpretation has been that it lasts for the entire encounter, but that does not logically follow from the rules nor balance.