The firmer the mattress is, the better it will hold up...(a simple physics rule of thumb). The longevity will depend on body weight. One sided mattresses will, generally, last about 5 years.

Density equals longevity. It's the difference between angel food cake and pound cake. The denser anything is the longer it will last.

Are you totally in to springs? Ever considered other technologies? Foam beds (latex and memory) or air technology beds like Comfortaire will better support you at a softer level. That means less back pain and less tossing and turning... But most people sleep on springs and replace them every 5 years because their backs hurt.