I alternate my road and off-road shoes and have 3 pairs of each as follows:
Pair 1: under 100 miles
Pair 2: 100-200 miles
Pair 3: 200-300 miles
As they're cushioned and to avoid injury, I bin them after 300 miles. Keeps me injury free, and to prove it I constantly run around 20 miles each weekend for marathon training and lots of other runs in between. Happy me!
Agree with pitstop crew aswell, advantages to rotation are drying the buggers out and allowing them to decompress.
Usually get bargains galore on sportsshoes.com and start fitness on the web so they don't cost too much.