Ask The Coach – Ten Golden Rules of PSA Rule #8

Jeff asks…
In The Ten Golden Rules of PSA you indicate Rule #8 is – All work is scheduled in 3 days in advance.
I understand that if everything is a priority then nothing is, but How do you arrive at the “3” days?
Also, I assume the 3 days out is for field service only? We also provide a help desk services for dental software. Our helpdesk remotely resolves a number of small issues very quickly.