I'm fairly certain we don't even have a policy. My personal practice is to change the short ones we use for venting qday, and the long ones for feeds when they get gross or I'm not sure of placement. Some people change them the second they walk in the door each shift, even if you tell them "I just changed it at 0500" and it looks fine. I've never seen anyone rinse and replace. The only ones that get left in more than a few days are ND tubes, and if we were allowed to change them, I have no doubt they'd get replaced at least once per day. We have a lot of, um, my-way-or-the-highway divas on my unit...