"You never make plans with a girl for further in the future than you've been going out." -- Barney Stinson, the Neil Patrick Harris character on How I Met Your Mother. (Neil Patrick Harris played Doogie Howser on the show by that name.)
This is an example of the "Copernican principle", which I've written about before -- if you assume that there's nothing special about now, you're equally likely to be in the second half of the relationship as the first. And if you're in the second half of the relationship, and you violate this rule, you'll have broken up before the plans happen.