when to clean barrol

Hi all 
I have the fx Royal 500 and it shoots like A Dream. But just the other Day when i was out 
shooting it lost accurcy. I cleaned the barrol
and pingo 2 Inc groups at 50 yards again. 
How often do i need to clean the smoot twist barrol ? I have never cleaned the barrol before now. Ca. 2000 shots sine i Got it last year.