My research tells me that a mattress is only good for 5-7 years according to The Better Sleep Council. The amount you use it does factor into this question and so if this mattress is not slept on everyday, the length of time this mattress lasts could be a year or 2 longer. The foundation that the mattress sits on is also important. If you had a bad box spring the length of time this mattress is good for could be decreased by a year or 2. http://www.bettersleep.org/onbettersleep/when_to_replace.asp
You're asking the right person, since I grew up around my mother's father, who hand-made and sold inner-spring mattresses and box-springs, for about 25 years! A quality mattress can "last" (stay comfortable and in good shape) for 20-25 years. The best rule-of-thumb is if, you start waking up with stiff neck or backache or you're having a hard time falling asleep or changing position several times during the night ... and you feel okay, otherwise... and the mattress you're sleeping on is already around 10 years old... look into replacing it. Keeping the mattress covered, under the sheets, with a plastic mattress cover will help it last longer... and wiping down the plastic, before each time you put different bedding on the bed, with a disinfectant liquid household cleaner (Pinesol, Fabuloso, etc.) will ward off bacteria and even insects that are attracted to upholstered furniture, like beds and couches. Also, turning the mattress over, about every six months, helps the stuffing hold up longer.
@Frigrl74 gives a great answer, in that it was thoughtfully researched, presented with a referenced, source, etc; good stuff!
However, I would say this really underestimates the lifecycle of a well cared for mattress. In my experience, provided the mattress is kept clean, is rotated (180 degrees) and flipped (one side vs. another) on a monthly basis, you can get many more years of safe use; 10 minimally, and up to 20-25 as @imwhatzcookin mentions above.