Act I ends when the protagonist(s) have reached a point of no return, which we feel comes when Mrs. Ramsay passes away in Part Two. Until then, all of the struggles of the novel (between Charles Tansley and everyone, between James and Mr. Ramsay, between Lily and all of the Ramsays) have taken place under the condition that Mrs. Ramsay was there to make everyone feel better. Mrs. Ramsay complemented Mr. Ramsay's rough edges, and her social ease drew everyone together in a way that could never be matched in the subsequent chapters. So the point of no return is the moment when Mrs. Ramsay is removed from the scene, and we are left wondering – how is James going to get along with Mr. Ramsay now that his beloved mother is gone? And how is Lily going to form her artistic identity against the discouragement of men like Mr. Ramsay and Charles Tansley without Mrs. Ramsay's (sort of grudging) support?
Act II should take us from that point of no return to the place in the narrative where the characters are furthest from their goals. Insofar as the characters have goals, then, we'd have to say that Act II starts with Mrs. Ramsay's death and ends with the beginning of Part Three, when James and Lily both pick up their struggles with Mr. Ramsay again. We don't know how James is going to resolve is problems with his dad, nor do we know how Lily's going to manage to finish her painting after ten years of waiting. But once Lily picks up her brush and James sets sail for the Lighthouse with Cam and Mr. Ramsay, we know they've laid the foundations of their final plot developments, which carries us into Act III.
This is the part of the story where everything is solved: James reconciles with his father, who has decided to share some of his power with James (symbolized by their shared arrival at the Lighthouse under James's steering). And Lily recognizes the contribution that the Ramsay family has made to her life without feeling like she owes them any shame over her unmarried status. Lily has solved her aesthetic problem of how to be a painter separate from Mr. Ramsay's philosophies on the subject. The End.