Becoming a dietician takes around 4 to 6 years which means you'll need at least 4 years to get your Bachelors degree in some places that require it and likely another 2 years of study to become a dietician and get your license to be the dietician.
So expect it to take you at least a minimum of 6 years to become a dietician but you may be able to become a dietician in as little as 4 or even 2 years in some places.