When do you give up?

How many times does someone need to decline your invitation before you stop inviting them?

My boss never brings his lunch so he is always looking for a lunch companion, and starts with me. I politely decline, as I always bring my lunch and he knows this, yet thinks he can convince me to go out anyway. It's not like he's paying either, and it wouldn't matter because[b] I Don't Want to Go Out for Lunch[/b]. Yes, I have gone with him on the rare occasion, but after years of asking I would think he'd eventually give up.