I set mileage goals. But when it actually comes down to riding, I look at how much time I have until my next commitment (or until it's going to get dark if I'm without lights) and I go for that amount of time. My average miles per hour is pretty much the same, depending on the bike I'm riding. So I pretty much know how many miles I'll get in given an amount of time. So for me, a mileage goals pretty much equates to a time goal since my speed is almost always the same. I have noticed I am slowly (ever so slowly) getting a bit faster, but not by any leaps and bounds. So for this 1000 miles goal, I know I need to put in about 71.5 hours.
I do agree with you riphraph, a time goal is a great way to do things, but miles just sound better.
PBW