from what the staff told me, this month there are a lot more people coming in to apple stores for battery replacement service. and how long you wait depends on how far back in the servicing queue your phone is for the day.
he said if you want to get the replacement done fast, come for the first slot of the day, because the later your appointment is, the longer your wait time for collection. he said that you can even sometimes see a short queue outside the apple store before they open, and these people queuing are mostly those coming in for battery replacement service.
oh ya. when i was there for my 1.15pm appointment on saturday, there was another guy at the same table as me who went for battery replacement service. he was just there slightly earlier than me (maybe 5mins). after the initial diagnostic checks were done, the staff quoted him a collection time of 6.20pm.. and when it came to my turn, i was quoted 6.55pm.. so i guess 1 battery replacement phone in front of you = about 30mins additional time before yours will be ready. but of cos the time they upfront quote you would be the "latest" time. i received the collection email at 5.18pm.
so TL;DR.. go first slot appointment if you dont want to wait more than 3 hours. and pray that they have stocks for the battery of your phone model. because if they dont, you would have to wait more than a day.