I do it every year, regardless of usage.
I run StanSan through the lines once a keg is dead, and leave the starsan in the lines until the next Keg is tapped......but
I have noticed that certain darker beers will leave the lines with a 'stain'
so just to be on the safe side, safe as in off flavors, I am not real worried about infection . Don't want any flavor from my Chocolate Vanilla Porter
making it into my Oberon.
I pull out the lines and trash them every January
I have also gotten into the practice of taking apart all the disconnects as well, and giving them a good soak, and a tooth brush scrubbing in Jan.
I do this in the winter, as I tend to run out of beer that time of year, from lack of brewing in Dec.
S