Probably 9:00-11:00am. You might be able to get a faster exit if you carry off your own luggage.
IF you carry your luggage off you can be ashore in an hour--- say 8.00 am.
Having cruised on Grand Princess this summer I hope your ship docks earlier than we did as we had booked express disembarkation which was due to start at 7.15am. We did not arrive in Southampton until after 8am and then discovered that QM2 and Navigator were both there too so taxis were very scarce for a good while. We did catch our flight home at 11am but only after being shouted out of the check in queue at Southampton airport. Hope you enjoy your holiday and make your flight.