Looking at the last tournament regulations update, and also the recent X-Wing tournament regulations update, it looks like both of them were set up to come into effect thirteen days after being released.
The first announced Regionals are May 13.
If they want to give us a new MWL before the first Regionals, and if they want to provide the same amount of advance notice, then they need to release the MWL by April 30. Which is a Sunday, so April 28 would probably be better for them.
That doesn't mean we will definitely get an MWL by then, but if I were FFG, I think that's likely the deadline I'd be aiming for.