I first went to a flat fee for unlimited time just before one recession. I gained market share & profits because others dropped their rates, their marketing, & wen to ala carte, nickel & diming them to death.
I've been toying with such an idea as that myself... it occurs to me that whether the event is booked for 4 hours or 6 I still can only do one event that day anyway...
What was your strategy with this and how was it presented?