To be honest, this really depends on the person and why the tube is being placed. If someone has a stroke and cannot swallow, they can often survive for many
years with a feeding tube.
People who get feeding tubes due to weight loss from dementia tend not to do well over time. Studies show that tube feedings in these patients do not really increase weight or reduce aspiration risk. Aspiration (swallowing secretions/ food into the lungs) can cause severe pneumonia, which are difficult to treat. Since these patients tend to be frail already, this type of pneumonia can often be overwhelming to their fragile system, especially if it happens often. Therefore, they can continue to decline despite having artificial nutrition.
Please speak to a health care provider you trust about making this decision. It really needs to be discussed, with the risks and the benefits clearly outlined for you.