From Makarska to Herceg Novi there is a travel distanced of about 210-230 km, during the summer season there the cities are connect with direct bus. Travel time with bus is about 4 hours.
If the direct bus is not suitable in relation to your travel plan, you can travel from Makarska to Dubrovnik and then continue to Herceg Novi from there on, do so should give your more travel options
Bus timetable Makarska to Herceg Novi, you can see here: