I'm a total train wreck! For X2, I made about 75% of my rooms over a two day period (exits and everything) and added more rooms that I needed as the story unfolded. I did not even have an idea as to how to end it, but it came together nicely. Comparing the two games (for me) I can tell X1 was built pretty much room by room because the hints/objects needed were pretty darn close to the puzzles that were in reference to, whereas X2 has a smattering of stuff all over and you need to discover how to properly use objects much longer distances away and/or much later in the game.
Furby McQuack on the other hand is a whopping ONE room large - that game took on a mind of its own and ended up being completely finished in an all nighter (4-5 hours if I remember correctly).
Enough of the shameless plugs...
Typically, I build as many rooms/objects forward as possible when I run into a programming problem that I cannot solve immediately. It's a nice little surprise when I get moving through an obstacle and discover most of the rooms and objects are already there and described!