Well, it had already happened to me, that someone asked me to look after his stuff and that he will be right back. And then… time went on. And I had my own business and couldn't wait (On a train, you have to get off, for example, that gives you no solution). What then? I've agreed to watch your stuff for a minute, maybe two, maybe even a few more. But not for half an hour. I would just leave, what else can I do (ok, ask someone else to carry on with the duty, silly).