My guess is July. I think the main feature of the Spring Update will be a Commons/Wizard City graphical update, so Part 2 can't come out then. I also think the Fall Update will be a 130 dungeon to culminate the third arc, so the only time Kingsisle can raise the level cap to 130 is in the Summer Update.
I know Kingsisle usually releases their worlds in the Fall, but they said that 2018 will "top all the other years" and that their list of plans is "long and ambitious" this year. I think this means 2018 won't be like the previous years in terms of content...
