For now, there isn't a fixed time: the empty-room event is random and has a 2-in-5 chance each time you check. If Robin is there, leave and try again; in the future, I plan to add a schedule showing when she's in her room so this is more intuitive.
Thanks and does dressrose ends after defeating diamante ?
For now, yes. The current Dressrosa content ends after defeating Diamante, but the story will continue in future updates.
