In prior times, buying Gates Green-Stripe hoses and belts, they lasted a good while.
How to tell when they need replacement? Belts start to crack on the inside, from flexing. Hoses usually started to seep at the ends, where the fabric reinforcement cords were visible. No "hardness" issues.
I always bought Gates Green-Stripe products. That was their best line back then, when everything they built was at least OEM-level. In more recent times, they have lesser lines of products, so get "the good stuff". NO NOS, NORS, or OEM items!!!!
IF tires start to "age out" at 6-10 years old, you might consider that as a decent gauge for belts and hoses, too. Looking for any date codes on the items purchased. In absence of such date codes, might figure on 4-5 years from time or replacement, as to a guide of sorts.
Seek out molded radiator hoses, when ever possible. Even if you have to wait a bit to get them, so plan ahead. One neat thing I've observed is that C-body radiator hoses are very close, if not the same, as B and E-body items.
Just some thoughts and observations,
CBODY67