I know from your other posts that you genuinely care for the health and wellbeing of your fish, and that you want to provide the best possible care for him/them. I second the advice given by AbbeysDad, especially his reasoning behind doing water changes. While aquariums are generally thought of as "closed systems" that mimic the "closed" natural habitat of a lake or stream, with a natural nitrogen "cycle " and an oxygen/CO2 cycle, in reality the lake or stream is not a true closed system. The water, as AbbeysDad pointed out, is renewed and refreshed by rain or watershed runoff, more so in some habitats than in others. New supplies of nutrients are introduced and waste products are taken away by outflow or broken down by natural processes that don't occur in the aquarium. (Keep in mind, also, the average aquarium is stocked at a much higher load per gallon than most natural bodies of water.) Thus, we must imitate nature's actions by adding nutrients and removing waste elements with periodic water changes. While it is true that one betta is a nearly insignificant bioload for a 29 gallon tank, as has been mentioned, there is a proportional amount of nitrification taking place in the filter and tank that will adjust itself to any increase in waste production. Also, there are those who will tell you that, being anabantids, bettas are adapted to live in poor quality waters because they can take in oxygen from the air they gulp in at the water's surface, which is true. However, the bettas we buy today are not the same sturdy fish that HAVE to deal with poor water quality in their native environment in order to survive; they are many, many generations removed from their sturdy ancestors, somewhat inbred (to develop desirable traits) and although still relatively hardy and able to survive in a small bowl, it is still susceptible to all the regular fish maladies. So do the biweekly water changes and feel comfortable in knowing that your betta will be one of the best cared-for bettas in the world, and if you must miss a water change or two, don't fret about it and just remember that, because of the size of his home, he will still have good water quality to live in.
By the way, a betta's lifespan is only about a couple of years or so, so prepare yourself for when his time comes.