I replaced the belt on my 2002 Saturn in 2011 with 220k miles on it, not because it was showing signs of wear but because I was going on a long road trip and thought it might be a good idea. I did not replace the tensioner pulley.
The serp belt on my 2006 Toyota Matrix started chirping on cold starts at 37k miles.
Really early considering my temperate climate, but there were lots of complaints on the 'net of that OEM (Bando) belt getting noisy.
Belt looked good but was hardened and lost grip.
Original belt on my 2005 Buick with 185k miles, in humid MS. I used to be more proactive changing belts and hoses, but they seem to last a lot longer than they did in the old days. Or maybe I’m just lazier. Or both, even!