If you only run 15 miles per week, you might not ever need to replace them. That kind of mileage just isn’t significant if you are an active person who spends a reasonable amount of time on your feet.
I used to only get about 250 miles for a pair of adidas Bostons (a cushioned shoe for my high arches) which amounted to a pair of shoes per month. Then I read something STer Ashburn said about running all his miles in a pair of 5 oz. racing flats until the sole was worn off. So I stopped buying new shoes and took six months to rebuild my mileage after a marathon. The result is that I no longer buy new shoes at all. I’m still working through all my old racing flats, spikes and one pair of air-cushioned trainers. I went 18 months without buying any shoes before I recently found a cool pair of trail racers that I wanted, but I didn’t really need them. And that is averaging 6 hours/45-50 miles a week over that period. I don’t spend a lot of that time on pavement though, and that helps.
So, if you are not having issues, I would not buy a new pair until the soles fall off. Be very aware of how your feet feel. I pushed a little too hard during one point of my adaptation period (basically about 5 1/2 hours in three days using old flats) and woke up with really sore feet and lower legs. I quit for three days, bought a pair of lightweight cushioned trainers and then continued at a more reasonable adaptation rate.
Chad