That is about the same as the question, "How long is a bit of string?"
It will depend entirly on the operator. Some places seem to change one fryer per day and others may only change once a week. It will depend on how good they are at managing temperature control and how much the filter the oil during use.
Easiest way is go in, buy a feed and ask them. Then look after them by taking all their oil and maybe even cleaning the storage area. If you collect more than you can use, put a note on the forum and I am sure others will take a bit of your excess.
Thing is most places will probably produce more than you need but someone else may come along and cut you out and new supplies are getting much harder to find these days.