Like Korbe, the most often I see is the "morning" and "evening" meal times / getting dressed.
I have had folks turn them on as early as 5 am (against rules) so they can get up and go fishing. My logic is if you're going to sit in a cold boat on a cold lake for hours, you can just get dressed in the cold too.
The sound does carry - even if it doesn't sound loud right in front of it, that hum can carry for quite a ways (think of highway noise and how far it carries).
I would prefer not to hear them at all, but I can accept rules 8-10 am and 6-8 pm without issue. I won't stay at places that allow them all day - if it's too hot to be outside than why be there.