Approximately an hour before lunch (in our example, that would 12pm for a 1pm lunchtime), call ahead to the company catering it if necessary (or you might have already sorted that out with a pre-order) and find a PA to pick up the order. You could go yourself, but I think it’s best that the producer stays on set at all times if possible. You never know when you’ll be needed.
Assume that by the time the PA returns, it will be just a little before lunch. Better be early than late with lunch.
And you can break early for lunch if you get to a point in shooting where it makes sense to stop (all the interiors are finished and you will be moving outside in the afternoon), but I highly recommend not breaking late. Or not too late. Everyone understands squeezing that last shot interior shot in in the morning, even if it pushes lunch a bit, but don’t take advantage of people’s patience.
Lunch break can be between 30 – 60 minutes. There are stricter times for lunch on union jobs, but just get a feel for how the group is doing and if you think they need the full 60 minutes, give it to them. If they seem to be antsy to get back to work after 40 minutes, then let them.
Lunch should not last longer than 60 minutes, though. If people are slow to get back to work, gently urge the group, and remind them that the later lunch runs, the longer the day will be. Or that it is important to get back to work because there’s only so much daylight left before sunset and all of the exterior shots still need to be shot.