Well if we want arena expansion to be superrealistic we should add some random "cost explosion" feature to it. Wouldnt it be uber-awesome if the arena expansion took like twice as long and cost four times as much as projected?
If we want it to be realistic at all, it should only happen in the offseason and should take "months." Last I checked you can't suddenly add 100 cheap seats to the top of most arenas or make a new luxury box appear in a week.