I think it’s all very seasonal; depends how busy etc... not that I have many data points:

I called up 3 weeks in advance whilst travelling F and got appts for my companion and I (half term in October)

I have also just turner up a few hours in advance of my flight on a Saturday in Y (last month, November) and my companion and I both got treatments 2 hours later back to back, albeit at T5B. After 2 hours in the CCR we were happy to go to T5B anyway as that’s where the flight was going from