We spent a couple days in Paris before heading onto Italy, while in Paris we took the hop on hop off city bus tours and bought a two day pass for two,we got off at one of the stops and when we tried to get back on we realized we were sold a used pass for one that expired that same afternoon,no ones fault but ours,so take a second and check your ticket in front of the driver or sales person.
I used one of these buses in Paris one year and got on the bus in late afternoon. At about 7:30 pm, after many long stops, I was kicked off the bus because that was the end of the day for the driver. Fortunately, I was able to figure out where I was and could walk back to my hotel from there, but there was no notice that I could see that the bus service would end so early. My only recommendation would be to board the bus early enough in the day that you have time for the complete circuit--at least two hours.