Woah, that's totally not right at all. A half gallon container should be 100% changed every day, a one gallon should be changed every other day, a 2 gallon every 3-4, and a five gallon once a week. Containers this small are troublesome to keep clean and troublesome to heat.
Bettas are tropical fish that need temperatures of 78-83 degrees to be comfortable, healthy, and active. A few degrees may not seem like a big deal to big warm-blooded animals like us, but to a 2 inch tropical fish it can cause digestive and circulatory issues, and ultimately lead to a weakened immune system. The metabolisms of cold-blooded animals are reliant on temperature, so I don't view heaters as optional to proper betta care unless you have a heated fish room or happen to live in a tropical climate without air conditioning. Most reliable and quality heaters are designed for containers that are at least two gallons in size.
Fish in small containers also tend to become obese and have shortened lifespans as a result of a prolonged lack of exercise. Many also become bored and develop neurotic destructive tendencies such as glass surfing and tail biting.
I'm not saying this to sound mean or preachy, but I think all of us have done things with our bettas that we've later come to regret. Some of the things that happened to my fish when I was just starting out could probably have been prevented if I had done more thorough research or someone had given me some good advice from the beginning.
Hopefully you will take the time to read this forum's article on basic betta care here: http://www.bettafish.com/showthread.php?t=49160