Helicopter rides for the general public or visiting tourists will take part throughout the morning into the late afternoon - even into the evening if there's plenty of daylight left.
Never will you take a helicopter ride at night for two reasons; one, you won't be able to see any sights if its dark; two, the night sky becomes more hazardous with less visibility.
However, if you've arranged to take a night time or romantic sunset theme flight then sure; expect to take a helicopter ride at night.
Under certain circumstances, there may be low lighting or it might be dark as you arrive back at the heliport after a long helicopter tour - such as a Grand Canyon sightseeing tour out of Las Vegas.