We don’t know when exactly Luke will leave the show, other than a vague rumor claiming that it’s planned for July, and an additional potential spoiler that indicates Laura will be staying on the show until the fall season. Whether her character will be kept for longer than that, we don’t know yet.
It’s entirely possible that she might be written in as a more regular character, but that of course depends on the schedule of the actress and the ability of the writers to work her into the plot on a permanent basis.
We don’t doubt the latter, but there might be some issues with the former, so we wouldn’t expect her to stick around forever. Still, even a brief appearance for a few episodes should be pleasing to most fans of the show who have enjoyed her previous roles when the character has been brought in.