I generally like to re-surface my rotors every other brake pad change.
Pad change one: De-glaze (DA Sander)
Pad change two: Re-surface
Pad change three: De-glaze (DA Sander)
Pad change four: New rotors
If I had the equipment to re-surface my own rotors, I would take the minimum material off when the rotors need re-surfacing the the first time and that would probably allow me to re-surface them a second time, but the guys at the autoparts stores usually take off the maximum amount of material so they don't have to make 2 cuts on your rotor (and be able to $ell you replacement rotors regularly).
1998 BMW 540i Sport
1989 BMW M3
1979 Triumph TR7 (1st car)
1986 BMW 735i (Sold)
1980 BMW 320i (Sold)