Is it normal for a tooth filling to last only 3 and a half years before it became deteriorated enough for it to warrant being replaced with a new tooth filling?

Although dental restorations are very durable and can last for quite some time, there are instances where they can fail sooner than later. This is not uncommon. Poor oral hygiene, bruxism, and accidentally biting on something quite hard can result in the need for replacement. May this be the worse medical issue you encounter in a long, healthy life.

Many reasons why. Poor oral hygiene. A bite that caused pressure on filling ,wearing it down, defective filling material and or placement, and too much acid in your saliva. Also grinding your teeth at night could cause a wearing down. Get it replaced to prevent further problems.