I was tested early in elementary school, and after starting in a program for 'gifted' children I had to be retested every year, and one year twice as they wanted to study the variance in two types of full intelligence batteries. Overall I found those tests to be far more informative than the ridiculous ones online or in cheapo books. A full intelligence battery will take between 2 and 6 hours to complete, depending on the exact type used.
In high school I was tested professionally again as part of a school project. Again in the military, and again while in college (wanted validated adult scores to use in gaining membership to mensa and other intellectual societies (hey, if nothing else it saves me money on my auto insurance)). I've found that my scores varied fairly substantially...moreso than is common based solely on age. Most peoples will vary dependent upon test type however.
If you really care, go have it done professionally. Otherwise, consider it a cute internet game and move on. Don't waste any money on such things however.