Old CRT (picture tube) televisions were unstable and so it was recommended to wait about 150 hours before calibration. Modern day panels are solid-state devices that do not have the same instability. My primary concerns before calibration are that you 1) are happy with the screen size you’ve chosen for your room and 2) ensure the TV is in good working order (not a lemon) before I arrive. Since I typically book a few weeks in advance, the time in between will be enough for these two decisive factors.