Apparently I’ve never been to a good bar lol. I used to ask places when I was first setting up my system. The answer was “never” or just a blank stare. I’m not saying that you shouldn’t have clean lines, but I certainly don’t think you need to do it every five gallon batch of beer. I certainly know that color isn’t everything, but mine are still as clear as they were when I installed them. Even if you did it every five gallons, it sure is better than cleaning 52 bottles. Have I mentioned that I hate bottling 
From my restaurant and bar experience, we had our lines cleaned every 2 weeks at the Bistro until we started using another vendor who used a different product that the state of Ohio said then only needed to be cleaned monthly. There was a tracking sheet in every restaurant where the tech would sign off when job complete. The form looked pretty formal and had state of Ohio seal on it. The people you asked just may not have known