I clean all of my lines every month or so and when I'm putting a new keg on. Low infection levels can take a long time to build up, but once they are established, every pint you pull will be sour.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk