Now there's a question!
The answer inst as simple as a time limit as a top end mattress will last a lot longer then a budget mattress. the Question should be "when is it time for a new mattress?" that is somewhat easier to answer. The first and most obvious is when it becomes uncomfortable, This can be down to not getting enough back support of just waking with back pain.
My second point is a follow on from the first, has something happened to your body or well being since you brought your last mattress? as an example I was in a miner car crash (stopped me from playing golf for over a year). The crash had an effect on my lower back and left shoulder, my body very quickly let me know that the mattress i was spending 8 hours a day on just wasn't right as i was waking with pain and taking pain killers to get me to sleep. I would point out i was at that time sleeping on a Sealy Posturepedic mattress which was right for me when i received it but no good after a small car crash. Sometimes you have to listen to your body, Its quite good at sending signals. If your waking up in pain after broken bone, hip replacement or pregnancy it might be time for a new mattress. I would also like to touch on mental health, I'm no expert but in my opinion a well rested body and mind can have a positive effect on your life.
Many clients of mine will often push for a time scale as other bed shops give a set amount of years. I like to say 10 years as a lot can happen in 10 years, you could lose or put on weight and generally your body just isn't the same as it was 10 years ago, however if your comfortable and sleeping well.....why fix something that isn't broken?
As a small note i would point out investing in a good quality mattress will in many cases mean the mattress will last longer, the pocket springs have a long life expectancy and allways make for a great choice.