It ultimately depends on how often you use your oven and what you're cooking in it. If you use it every day for every meal for a large family, then your oven is going to accumulate spills, food debris and grease in it more quickly than the oven of someone who only uses it occasionally.
It will also depend on how often you, yourself, clean your oven. Most people don't have the time nor the desire to do a major oven cleaning themselves. Some manage, at least, to clean up obvious spills. But no matter how careful you are, there will still be the spills, food debris and grease that end up on the surface of your oven and get burnt on more and more every time you use your oven. Grease spills, especially, can release foul smelling smoke every time you use the oven, and have the potential to break out in flames that could damage your oven or home.
We recommend you have a professional cleaning at least once a year to keep your oven working well and staying safe and hygienic.