I never gave this much notice before, but there was definitely a timeskip between Zhan Tiri's defeat and when Cassandra left Corona for good. The main hint being the castle itself since during the final battle, it was heavy damaged in various areas; Rapunzel's bedroom, hallways, the Throne Room and the entrance to the castle.
But during the reprise of Life After Happily Ever After", it's shown the castle has been almost completely repaired with Rapunzel's room being the only exception.
The repairs could NOT have happened overnight. It takes time to get things fixed and restored so my best guess several weeks to a month must have passed between Zhan Tiri's defeat to Cassandra's departure. (or longer?)