Honestly, it varies. I didn't check how many miles before I changed my deck, but it was about 3 years of fairly hard running on it, probably averaging about 20mpw on it (which means I went strectches with 0-10 miles and some with 40-50).

As said, my deck was probably actually fine to go - it had mild cracking on the surface which I only noticed because I was in a barefoot running phase and could for the first time feel them under them belt. It was very superficial when I took the deck out, and for sure I could have even flipped the deck for it to keep going. I only changed it because when I called Sole to ask if this was a problem, they just said they'd send me one for free under warrantee before I ran into any issues.

My 2nd deck change was again, an 'optional' one - I was replacing one of the rollers which had rusted out and they sent me a 2nd deck fwhen I mentioned the new deck had some minimal fissuring on it as well about 3 years after the first (out of warrantee). I do think I paid for that one, though (they sent me a new motor for free as well at that time.)

It varies with how heavy, how hard, and how fast you run, but it should last you years. I'm actually positive I could have kept running on my decks despite the damage, with no problem in retrospect.