Oh my the scary question.

Most places legit never do unless there's a problem. When I do service I'll usually flush what is needed but I don't have time for a full cleaning since the customer is supposed to do that.

Corporate chains tend to have actual SOP so they tend to be cleaner in general and will have their lines cleaned more often.

I generally suggest every 4 weeks, and to do it when a box needs to be changed since you are only gonna be flushing the last bit of an old box out of the lines. Luke warm water in a 5 gallon pail and you dunk your qcds with the caps off, run the gun or fountain till it's clear then start mashing the buttons to get any bits stuck in the line. Pull the qcd from bucket then put cap back on, reattach to box and run gun/fountain till product is running properly.

Tbh, it's not the lines you gotta worry with soda it's the gun itself, some places never clean the nozzles and they get gross. Also most of the time any bad tastes are related to water or a bad syrup ratio. I had someone call me saying their lemon lime tasted "dirty" it was a bad ratio, took me not even 5 min to fix.

I'd worry more about the beer though that's a whole different beast. Lines should be done every two weeks, once a month at the very least. That shit will make you sick if it's dirty enough, I've seen so horrid things related to beer.