Used to go by the 500 mile rule, but decided a while back I'd give it a whirl and see what happened if I kept using them. Got over 900 miles out of 2 pairs, and am currently on 700 miles on my most recent pair. They don't feel quite as plush as they used to, but no problems. So therefore have now saved the cost of 2 new pairs. I'm 12 and a half stones and usually use a support shoe.
If they aren't causing problems, why change? Once you start getting niggles, then consider it.
TBH running on short, level grass which is dry and firm will probably make very little difference compared to running on tarmac. Any undulations and different wind speed will probably have a much bigger effect. Of course running through thick or slippery mud or overgrown grass would be rather different. A proper synthetic track will obviously be fastest of all.