It's the quality of the materials used and how they're put together that determine how long a foundation and mattress set will provide proper comfort and support. The best assurance of good performance over a good many years is to buy the highest quality sleep set you can afford.
Don't look to the warranty to tell you how long to keep your mattress and foundation. The warranty is there to protect you from product defects, not against the gradual loss of comfort and support. Keep in mind that while a mattress set may still be usable after many years, it's probably not giving you the good night's sleep you deserve.
Mattress Size
Cramped quarters can turn sleeping into a nightly wrestling match. A healthy sleeper moves anywhere from 40-60 times a night, including some dozen full body turns. You need freedom of motion while you sleep and to help you relax while you're falling asleep.
If you sleep with a partner, be sure to select a queen or king size bed. Both are not only wider, but several inches longer than the standard "double" (full size), which offers each sleeper only as much space as a baby in a crib.