You can control the persistence time of a message on the noticeboard using the tag <Duration=hh:mm:ss>. However the bit about scrolling back I'm not sure I understand. Do you mean you want a new message entry each time the object changes value? I.e so you have something like the history list in Cortex for that object? If so then that is not the way the notice board works. A given object will update its particular message rather then append a new one. If you look at the example screen shot of the notice board connected App in my earlier post, you will see a number of different data items being displayed with times adjacent to them. So as an object changes state or some other reason to update its status (could be scheduler driven or on a button press or any other signal) then it is just the object's data that gets updated rather then an entire new entry. So you can see at a glance for example when the door was last opened or closed or you can see how long something has been going on for if you use a time accumulator, that sort of thing. So the idea is that it is a notice board not a history list display - i.e not something you'd normally want to interact with to scroll.
An app could however be created which simply appends new messages to a list (possibly a simple modification to existing one), but I imagine this would only be used for a single object otherwise the list would be cluttered up with all sorts of data items.