Work backwards from the guests
Start with the hours guests are actually present. For a shower, two to three. For a dinner, three to four. For a wedding with a ceremony and a meal, five or six. That is the middle of your booking, not the whole of it.
Setup takes longer than you think
If the venue sets the tables and chairs to your floor plan before you arrive, you need roughly an hour for your own things: flowers, place settings, signage, the cake, whatever you carried in from the car. If you are arranging the furniture too, add another hour at least.
Two things always take longer than planned. Flowers, because arranging them in a room is different from arranging them in your head. And anything involving a ladder.
The clearing is real
Allow forty-five minutes to an hour at the end, more if there are gifts to carry out. This is the hour that people skip when they are pricing a rental, and it is the hour that turns the end of a good evening into a scramble.
The buffer that changes everything
Add thirty minutes you do not plan to use. Guests arrive early. Deliveries arrive late. Something needs a second trip to the car. The difference between a host who is greeting people at the door and a host who is still lighting candles when the door opens is usually about half an hour.
A worked example
A shower for twenty-five, starting at eleven. Guests present until two. Setup from nine-thirty, clearing until three. That is a five-and-a-half hour booking for a three-hour party, and it is the right amount.
When a longer block is worth the money
If you are decorating yourself, if you want photographs in the space before guests arrive, or if the gathering is one you would rather not watch the clock during. The extra hours are usually the cheapest part of a rental, and they buy the thing people actually want from a venue, which is not feeling rushed.
Tell us what you are planning and we will work out the timing with you.