I have my H20 in a room that doesn't have a telephone outlet, and I use the receiver infrequently. There have been a number of times I've had to reauthorize it when I first turned it on (because I hadn't used it for a while), and there have been times that it stayed authorized for a month or longer, without my intervention. From this, I would guess that the de-authorization period is at, or over a month, but as NR4P said, it doesn't matter since a quick call to Directv will get it back up and going, and then it's up for an unpredictable period. Fortunately, it's not an issue that happens frequently in any case.
A helpful note, when you call Directv, give the automated answering system the screen error code and it will do the reset. Otherwise it's real easy to get lost in their menu, and you end up needing to talk to a CSR for the fix, which I'm sure is a waste of their time.