My rule of thumb - when it quits working, clean it! The gun will tell you when it's time through bobbles and malfunctions. Obviously, under adverse conditions, this is subject to need.
In 12 years military service, I saw more good rifles ruined through excessive and or improper cleaning than from any other cause. The usual dictum of a 'light coating of oil' meant that you had set the rifle up for accumulating dust, dirt and possible malfunctions. The folding cocking handle on our FNC1's was famous for locking shut, requiring a bayonet to pry it open. Under winter conditions, it was prone to freezing shut, again, requiring a bayonet to pry it open.
Most civilian rifles are never going to be subjected to the #### and abuse that is routine for a soldier's rifle, or be used under such harsh climatic conditions for prolonged periods.
Current cleaning solutions and oils are far better now than ever before, providing for better lubricity and rust protection.
One of the most sought after lubricants by American soldiers is CLP, always in short supply where needed, and often requested in 'care packages' from home.