Game dart players may play all day long (5,3,5)
I believe the answer is:
round-the-clock
I believe this is a double definition.
'game dart players may play' is the first definition.
(variety of darts)
'all day long' is the second definition.
(I've seen this before)
(Other definitions for round-the-clock that I've seen before include "Twenty-four-hour (operation)" , "24-7" , "From midnight to midnight" , "24 hours without a break" , "For the full hours" .)