Keezer lines spoiled?

Hey everyone, I have a question as this has started to become an issue I've decided to recognize more. When you've finished a keg, are y'all cleaning your line immediately after with sani water/ other means? Do you put another on immediately after? are you changing the lines even if the same style calling for same line length? I've noticed on my last few kegs that my lines are becoming "stained" even after I wash through with sani water right when the keg is finished. It's not that big of a deal changing lines each time, but I'd rather not. Secondary question, how long do you leave a keg on a line before the lines go "bad"? If that's a thing.