Indeed, I agree. Wonders should really be built with their primary benefits in mind, not the side benefit of the extra GP points.
Personally, I think it's more when you have the luxury of choosing and picking locations for each world wonder that you tend to spread them out in different cities (to avoid contamination of Great Person points you don't want, for instance). On the other hand, when you're playing in a highly competitive game, you usually won't have a choice of cities to build wonders in. You'll often have just one city that happens to have good enough production to build your wonders, so the same city that builds your first wonder will tend to be used in attempts at building other wonders (at least in the early game).
That's my experience from multiplayer games, anyway. Of course, in single player things are different, and you might be able to afford having multiple cities build different wonders in the early game. (All this is assuming you're building wonders at all, and aren't rushing out an army instead.)
My single player games usually last from a couple of hours to a few dozen hours. (I had one game that I spent close to 100 hours on.) That'll usually be over the course of 1-30 days, depending on how busy I am.
Multiplayer games I'll spend many months on, though. Some take over a year to finish. It's great fun, as long as you have the patience for it!
Indeed - each new level requires 3, 5, 7, 9, 11, 13... extra XP.