Number of miles is not such a good guage, type of driving is better...
If you are doing very short journies your oil will deteriate quicker as your engine is not fully protected until it is warmed up and the metal on metal action will cause more crap in the oil.
If you have any blowback in the pistons then acids get into the oil and this deteriates it more, so you would also need to chagne it sooner.
Some say follow the manufacturers specification, others change when they feel like iit, others do it once a year. Some just wait until the engine dies.
I personaly end up doing it every time i end up taking my engine a part (every year, cos I keep finding oil leaks) and then about 500 miles after that...
Simon