In a standard corporate environment with PCs they are replaced approximately every 36 months on a revolving schedule.
In introducing Macs into the corporation, the IT department appears to be unable or unwilling to make a schedule for standard replacement, and thus the Mac users are getting upgraded ram and such, but their machines are older than the PCs.
What information should we be taking to the IT department to convince them to create such a schedule? Should we expect the schedule to be longer than typical PC replacement schedules, or the same? What aspects of a Mac make this a different equation than a PC?