An exact start date for the event has not been announced, but expect it to start in the middle of March–right around the one year anniversary of the theme parks closing.
Assuming that there isn’t another significant delay in that.
With expectations that the parks will finally reopen by sometime this summer, I’d expect the plan may be for this event to last until it needs to close in order to make the parks ready for a proper grand opening.
Exactly how long the event will last has also not been announced, but the Disney California Adventure Food and Wine Festival, which this new event will essentially be, lasted from late February to late April in 2020.