How long should a restaurant hold a reservation for you?

A reservation is a promise between the diner and a restaurant. You agree to show up at a certain time with the members of your party, and the restaurant agrees to have a table ready.

The idea is clear, but the reality is a bit muddied. Sometimes you have to wait as a party sits with an unpaid bill on the table. And sometimes you’re running late. Which raises a question: How long does a restaurant have to hold a reservation?