I've got a few that I've had for a couple of months, one on a carbon gooseneck gets hammered on upvc cleans with really hot water. They have all gone out of shape to some extent, I think due to the hot water but they all are still the best cleaning brushes I have.
A friend got one that has had a lot more use than mine, as it's his only brush, but he's got cold water and his brush hasn't gone out of shape at all.
Simon.