Your Pilot has a maintenance minder on the instrument panel that will indicate when the differential service is required. Look for the number '6" to illuminate to change the rear differential fluid.
HOWEVER...if you tow a trailer, live in a very mountainous region and do a lot of low speed driving, Honda recommends replacing the fluid first at 7,500 miles, and then every 15,000 miles thereafter.
Rule of thumb - if in doubt, do the extra maintenance. You won't be sorry.