I can usually manage to get 7 days - have gotten 8 days once or twice. I know of people who regularly get 10 days or more without a problem. As long as there is no sign of leakage or skin irritation, go for as long as you can I say. Sounds like your patient is one of the luckier ones to get such a long weartime.
I have had an ileostomy for 30 years now due to UC.
I'm not a complete idiot - some parts of me are missing!