Depening on caliber, get a military issue headspace (timing gauge also for FA)
gauge.
There is no way to screw this up if you do it according to the TM and have the above items. Doing it this way ensures that you will not hurt yourself or other people near the weapon when firsed.
How often to check it? Check the headspace WITH A GAUGE and do a function check before you shoot each time. After this is done, your will fire the weapon.
Depending on your ambient climate temperature, humidity, ammo used and cleanliness of the weapon, always re-check the headspace after 100rds. Do that sequence for at least 3 cycles. If you are in extreamly hot weather, such as Iraq, you will find that your headpace will change as the sun warms up the weapon quite quickly and the headspace WILL change. Yes, you must contact the TC VHL CDR and tell he/she that you are UNLOADING the weapon and re-checking the headspace and do a function check. Once reloaded, tell the above personnel that it is reloaded, headspace and function checked and back in service.
My lube of choice: Remington Oil wiped on with a thin coat of LSA medium weight, or light weight LSA in a pinch. Do this on all the bearing surfaces inside the receiver as well as the bolt, barrel extention and barrel threads. Then "relube" with keylock graphite on all these areas.
This has worked for me for many years in high humidity, low humidity and two wars in the Persian Gulf countries.