You may accumulate surface rust on your rotors yes, but having to re-replace them within six months is extremely unlikely unless your rotors are uneven and causing undue wear on your brake pads.
Rotors can rust, more so if the vehicle is not driven at all. If the vehicle is driven regularly, the heat created by each brake application usually dries the rotors very quickly and reduces the possibility of rust. Even if the vehicle was not driven at all in 6 months, I would not think that would be a significant time for so much rust to build up that it would cause a problem. At the very worst, maybe the rotors would need to be resurfaced. However, there may be circumstances that possibly were not properly explained about this situation. It never hurts to get a second opinion. Also, remember to always ask to see the parts that the technician is referring to and ask for a complete explanation.
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〉 Answered on Mar 22nd, 2009 by Suzanne Grego, Technician at City of Philadelphia Fleet Management