depends on the mileage.
training 700-800 hours a year (my 20s) – new shoes every other year, or every year if there was some shop sponsor discount
training 600-700 hours a year (30s, 50-53) – new shoes every three years, maybe every two.
training 500 hours a year of less (since 53) – every three to four years.
I have kind of funky feet (wide metatarsals, long flat arch, narrow midfoot), so even with insoles no stock shoe really fit me well (I always wanted to saw some Carnacs and Sidis in half, and stick the Carnac toe box on the Sidi midfoot and heel), and I went to custom shoes in '95 when I found out about some US makers. Given the cost of custom shoes, I’ve always tried to wear mine as long as I can. When they’re shot for the road, I caulk them up and use them on the trainer.
