It took us 7.5 hours at Croydon in June & just be aware that booking an appt doesn't really mean anything other than they are supposed to know that you are there

Everyone who gets in & pays before you will probably be seen first even though you've been given a specific time.
But now I've just realised that you probably mean just how long did it take in that specific office. doh
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I do think it is best to give yourself a goodly chunk of time, though. Immigration has a huge monopoly, IMO & seem notoriously unaware that when paying for a premium service, we sort of do expect just that!
Sounds like a good excuse to see what Liverpool has to offer! Hope your experience is better than ours was!