If you want to know exactly the commercial fryer oil changes, it depends frequently on the professional who assists you. This kind of opinion can vary depending on each person. since your employees to the call centers o specialized companies, everyone has different opinions.
However all this answers depend on multiple factors, like the type of oil you use or even how often your kitchen service use the fryer. Also there are many ways you can extend the life of your fryer oil just by making the most of it.