User perceptions (and expectations) of backups
One of the reasons I think we can go with our planned backup schedule is how I think that users will perceive it. This matters because unless you're lucky enough to have a specific backup schedule mandated from high, part of setting up your backup system is managing user expectations of what you can deliver and making sure that they feel that it's reasonable.
(Let us take it as given that you cannot just ask the users what they want and then deliver all of it. Users don't want to have to think about that sort of stuff, and if you push them to do so anyways they will tell you that sure, they'd love to have daily backups that go back five years. Because, really, who wouldn't?)
Specifically, my sense is that the further your backups go back, the less 'precision' that people will expect, the less they will expect you to be able to get back a file as of a specific day. Intuitively, it seems far less reasonable to demand that your system people be able to restore your files as of exactly one year, three months, and two days ago than it does to demand that they be able to get back your files as of last Tuesday.
(Partly this seems intuitive because in general recent events in are more vivid and more precise in people's minds than more distant ones.)
You can of course shift your users' intuitions and train them to expect your backup system to be more precise than that. Just be sure that you can deliver on it.
(We never discussed specific coverage with our users, partly because it was complicated, partly because we knew it would change over time, and partly because very few users are actually interested in that, or care enough to demand that it be officially documented and promised.)