persists, should you lie....? ....About what time you are planning to meet?

I had plans to meet my friend at a specific time and she was exactly 40 minutes late. She's been 30 minutes late before, and 20 minutes late before....I asked her why she was late and implied that I was tired of waiting, but she kinda laughed it off and apologized.

So next time, should I tell her to meet me at least 20 minutes earlier than I plan on meeting her? Like if I want to meet her at 5:20, should I say to be there by 5? That way she would get extra time to be late, I wouldn't have to wait that extra time, and all would be merry. Unless she ends up being early for once, which I doubt would happen anyway...
To make it more annoying, I'm usually 20 minutes early, I would hate to be late...