Dentures do last for several years but exactly how many can vary significantly from person to person. It's worth pointing out that there is nothing stopping a person from wearing a denture that has worn out long ago so you can, technically, make them last as long as you want. In the same way, you can use a set of tires for your car long beyond their recommended mileage but it will compromise performance and safety the longer they are used.
When we assess a person's denture we generally look at age, fit and condition of the teeth. Typically, we find our patients get 7 to 10 years of useable life from their dentures. Relining a denture is an option if it is, otherwise, in good condition but the gums have shrunk in the last several years and the denture feels loose. If the teeth on the denture, especially the back teeth, are worn down significantly it's time for the denture to be replaced. The back teeth do the lion's share of the work and all those bumps and grooves on the bicuspids and molars are crucial for being able to properly crush and tear meats, fruits, vegetables, nuts and grains.
So, how worn down should you let it go? Well, getting back to the tire analogy, when a mechanic assesses a tire they talk about minimum tread depth. The tire still looks like it has some tread visible and should do the job but it is not enough for proper performance. Back teeth have raised sections and grooves similar to a tire and when they lose a significant percentage of that "tread", they will not be able to grip, grind and tear food nearly as well as they did years before.
Because this wearing down process takes years, it's sometimes difficult for people to assess how much their chewing ability has deteriorated.
Doing an assessment on someone's denture takes just a few minutes and is free so we urge denture wearers to take advantage of the opportunity to be able to come away with up-to-date information for future decisions on their oral health.