I like to ask this every so often as our service intervals tend to vary depending on the season or the oil that we are using.
Right now, mine is every 7500 miles with 5W30 or 10W30 Formula Shell conventional oil on the Toyota van and the Saturn.
Totally agree with DragRace above. These days the raw material quality is so improved that drain periods can be extended. A BMW Longlife Oil can last for 18,750 miles. DaimlerChrysler has specifications like MB 229.1; 229.3; 229.5; 229.51; 228.1; 228.3; 228.5 and 228.51 and oils meeting these specns can last for 18,750 - 25,000 miles. Fiat approved ACEA specification can go upto 15,625 miles. Ford approved oil can be used in Ford cars upto 12,500 miles. Opel has an approved longlife oil which can go upto 18,750 miles. Porsche approved engine oil can last for 12,500 miles in Ford cars. PSA (Peugeot-Citroen) has the PSA approved oil that can last till 9,375 - 18,750 miles. Renault recommends the use of ACEA A3/B3, B4 level engine oil similarly. VAG (VW, Audi, Seat) approve the VW approved oils (standard service) till 9,375 miles and Longlife service till 18,750 - 31,250 miles.
I've worked up from ~5K to 9.5K. Living in S. Az, there's no real need for a special winter oil, so the OCI are entirely mileage based.
Running syn (mostly M1 10W-30 mixed with ~15% 15W50EP) for the last 4 years has cleaned my used Honda's engine to a phenomenal degree. When the oil is still a semi-clear amber color at 6K, I have a real hard time tossing it!
I appreciate that color and 'age' don't correspond 100%, but various UOA indicate that M1 and other solid syn oils are entirely viable to 10K and beyond. With low blow-by, I'm not as worried about acid, soot, and other stresses on the oil. I'll have my first UOA back soon, and will adjust OCI accordingly.
Wife's car is 2 months,short hopper and wicked bad fuel dilution/burn off. I've tried all kinds of synthetics and dino's,best to hold up so far is Mobil 5000 5w30.
SIL's car, every 6 months, M1 10w30 HM or @ 7500 miles.
Daughter car, every 6 months M1 0w30.
My truck, too soon usually.
In my Ford engines I shoot fo 10,000 mile OCIs. I've been doing that for 31 years with M1 oils and has worked well with many high mileage engines.
My sons 91 Ranger has 330,000 miles on M1 with 10,000 miles, and sometimes more, OCIs.