Usually once every month or two will suffice, though it depends on several factors, including how often you use the knife. Some knife designs and blade steels will stay sharp longer than others. Additionally, different cutting tasks will dull a blade at different rates. For example, cutting into bones will dull a knife much faster than cutting vegetables.
Which is better, carbon steel or stainless steel?
Stainless steel (any steel with more than ~12% chromium) is highly resistant (though not immune) to rust and corrosion. This makes maintenance much easier for people who are not used to caring for a knife, or do not have time to.
Carbon steel will rust easily if not oiled and kept away from prolonged exposure to water. It will also develop a patina with use, which may be undesirable to some people, though the patina is not harmful like rust is. On the other hand, carbon steel tends to outperform stainless steel in terms of edge retention, sharpness, toughness, and ease of sharpening.
In summary, stainless steel is popular with most knife owners, while carbon steel is typically preferred by individuals who prioritize performance more than ease of maintenance.