It will vary from person to person.
I had notions of running, bought some decent trainers did a pregnancy test 2 weeks later and two babies later, (so after about 3.5 years) did C25k, so the trainers were a bit elderly and knackered. I can't remember when I replaced them but it was probably roughly around completing when I realised I actually quite like running so was worth it.
Weight puts more pressure through the shoe, as do "odd" gaits. Some people are more prone to niggles and injury. If you're happy and feeling good, I wouldn't rush for the sake of it. Up to 5k a few times a week is a fairly light load.
In a typical year where I'd race 2-3 HMs I'd get through 1.5- 2 pairs per year, but I'm a niggly runner, in need of support and while I'm not "heavy" my feet are dispropotionately small for my build which is pressure on the shoe. I tend to find that niggles (such as tired shins) creep in around the same point that wear patterns show on the soles, and uppers look worse for wear. I tend to then semi-retire the old pair for walking around and get another year out of that.