Wood movement is 100% - if it's wood, it moves. To what degree depends on a few things - width, thickness, humidity differential (what it has vs. what it's surrounded by), and sometimes temperature. Temperature's usually not a problem, but humidity certainly can be. The width of your stock certainly plays a very large role in how much movement can happen. Generally, I expect as much as 1/16" for every 12" of width. It also depends on species - some will swell up more than others. The Dept. of Forrestry (i think) has a bunch of information on this. Google "Wood as a structural material". That will give you charts and tables on just about every aspect of wood.
The other thing is - there may be a very good reason you haven't noticed anything with the wooden things in your house is that they were built with movement in mind. Properly done, nobody should ever know. The exception to this is in some styles where a breadboard end or some other cross-grain situation exists. Another possible explanation is that all the parts that COULD suffer from movement are made of plywood or some other veneer or movement-neutral material. Plywood and veneers over MDF and such are very stable and don't generally move much - which could also explain why you haven't witnessed any appreciable movement.