In case you have to back up your save files for Spec Ops: The Line, by default, they can be found in the following location:
C:\Program Files (x86)\Steam\userdata\{########}\50300\remote\
Note: {########} will be a string of 8 or 9 numbers which are unique to your Steam profile.
The exact directory may change slightly, based on where Steam is installed on your machine, but the folder location should remain constant, relative to the main Steam directory.