From the 747 FCOM:
"If using autoland, verify the autoland status at 500 feet radio altitude".
From a letter from Boeing in response to a question about latest allowable AP engagement for Autoland:
"Provided that an ILS that meets the autoland criteria, has been tuned, the airplane is being manually flown on the published ILS path in trim and on speed, and APP has been selected prior to 1500 feet AGL, selecting the autopilot prior to 600 feet AGL (which is the lowest altitude for LAND 2 or LAND 3 to be annunciated) should satisfy the logic for autoland. The time required for the autoland system to perform a self-test and confirm correct operation will vary depending on conditions. We recommend that the crew engage the autopilot as soon as possible in order to satisfy approach logic.
Note: The autoland status must be confirmed by the crew at 500 feet AGL.
Note: Late autopilot engagement is not a recommended practice."