My in-laws have never been less than half an hour late. I've learnt to make something in the slow cooker or something I can produce in less than half an hour.
When we go out for dinner I always offer to book it and lie and tell them 15 min after I actually booked it. Still doesn't always work. We've just tried to accept it now, we have a wee bet on just how late they will be.
Phew, sorry for the rant! If we know what time we are eating I'll be there 1/2 or and hour before to chat and help out. It is rude to turn up and expect food as soon as you arrive.