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The timetable on the Opstin Kotor website shows the last bus leaving Kotor at 00.15 and leaving Kostanjica at 01.15 (takes apprx 15 min to get to Perast) for Kotor. I've never been out that late to prove it's existence but I have seen it around the 11.30 mark.
Forgot to mention that is the Monday-Saturday timetable. Sunday, the bus goes every two hours (rather than hourly during the week). It leaves Kotor on the uneven hours (eg 9.15) and leaves Kostanjica/Perast on the even hours (10.15/10.30)