>I would consider adding new property CharactersLeft to the ViewModel, binding the label to this property, and changing this property inside the setter of a property that the textboxe's Text is bound to.

That is what I have, but the setter property of the textbox binding does not get used by WPF. There needs to be another way to hook up an event so I have updates from the Texbox changed within a view model.

I can read the length from the bound property easy enough, I just don't know when to read it. I have the label bound to a property that returns the value needed to display in my View Model. Works fine, just doesn't update

I am wondering if there is a way to hook up a binding directly to the textbox element and get length.