I arrived at 10:15 for a 1pm flight (actually got there at 10am then wandered around the terminal trying to find Histiopath before realising they are in a tent opposite the terminal on the departures level. They say 3-4 hours, but if my experience is the norm then 3 is more than enough. You don't book a spot per se just give your passport and flight details when you book online then turn up whenever you want. The nurse scans your passport, which gives them all the details you have pre-entered, and then does the test. The data collection is all automated which saves a tonne of time. It's like a production line in there. I was surprised how quickly they were moving people through.

ETA: I could be wrong, but I don't think the airport locations accept walk-ins. Probably because of the data automation.