From the 744 FCOM:
The FMC transitions to "on approach" under the following conditions:
• a VFR approach is created and: the airplane has sequenced the FAXXX or the airplane is enroute to a direct-to or intercept course-to the RWYYY waypoint and the airplane is within 25 nm of the runway threshold
• a published instrument approach has been selected and incorporated in the active flight plan and the airplane has sequenced the first waypoint on the published approach
The approach condition may be delayed if the flight crew manually inserts, bypasses, or deletes an approach waypoint on the LEGS page.