The short trips are worse on the oil than long extended drives. But I would have to agree that 1500 miles is crazy.
If you really...really want to know how long you can extend your oil change you have to get a sample tested. Be sure to get a TBN test. That is the only way to tell how much additives are left in the oil. The other test is only a wear test.
You can get testing done here.
http://www.blackstone-labs.com/
Danny