Someone will come in here and quote an owners manual from 15-20+ years ago and say 60,000 miles, which is ridiculous on everything but super special cases. Serp belts these days being made out of EPDM rubber last 100,000+ miles easily.
The camry in my sig is running the original belt at almost 300,000 miles. My previous truck had 200,000+ miles on the original belt. Unless you have a very special scenario I wouldn't even dream of replacing one until the 125,000 - 150,000 mile range.