2,000-4,500 miles rear, up to 6,000 miles front, depending on tire and load. Replace when the outer, hard rubber is gone and it starts flatting, or when cords are visible.
I collect leaky tubes and patch them at home, or on the road when I've exhausted spares. I think I'm up to four failed, repaired tubes after 20 years or so.
I've limped home on two tire boots, and had three other booted tires failed; it's worth a try, but replace a blow-out ASAP.