how long ""?
i would think near instant.
hook up the pw, goto the gateway config installer wizard. step 5 (powerwall setup). once config'd, its done locally. then on the next cloud sync it will push the new config to the cloud api, which is in seconds if you have an active connection.
STEP5:
"Powerwall
Listed are all the Powerwalls auto-detected by the Gateway. Scan again if Powerwall is not listed."