I don't think there is a set requirement. Do you keep it inside or sitting outside? condensation, corrosion and fuel bacteria growth are the main issues.
If you start it up, run it to full operating temperature to evaporate any condensation in the oil, and ciculate oil to the higher parts of the engine. drive it to allow oil to "climb" up the gears that are above the oil level (PTO and ring gears in differentials come to mind. Also park with different part of tire on the ground each time.
Because the transmission and hydarulic tanks are vented, condensation can accumulate and make these oils appear milky so check and replace as necessary, regardless of hours used.
Use fuel additive to kill bacteria in diesel fuel and store with tank full to prevent condensation problems (fuel contamination and rust).
I suggest a battery tender be installed so when you do get ready to go, the battery is hot and ready.
Also, clutch disks can get stuck to flywheel and/or pressure plates so it is a good idea to cylce the clutch. on your cama the hydraulic pump will put enough load to keep the PTO disk working, on other tractors you may need to have an implement on the back for some resisitance against the PTO disk.
I guess I would say, try to run it monthly or so.