If the vehicle in question is a 2010/11 Legacy with a 4-cylinder engine, the first oil change (just like all of the succeeding ones) is listed by the mfr for 7,500 miles. I would recommend that you change your oil on a regular basis at ~4k miles.
If you opted for the superior 6-cylinder engine, then the mfr is VERY specific in stating that the first oil change is due at 3k miles, and then every 7,500k miles thereafter. I changed the oil at 3k and then reverted to my usual 4k schedule.
If you don’t believe me, the same information can be found in the booklet titled Subaru Warranty and Maintenance, and it should be sitting in your glove compartment.