One way to deal with the "stale data" problem is to prune old users from the system, but many people will be understandably dissatisfied with this, and it is not recommended.
I don't think you should worry too much about this. Someone who abandons a free service for an inordinate amount of time shouldn't expect to see their data. Especially when the cost of storing such data isn't nominal.
Send them one or two reminders and you should be fine.
Yep, that makes sense. We're actually just implementing this now, hopefully it will solve our problem (it looks very promising, from some initial data crunching we ran), and is very easy to implement.
We plan to send an email one month after the user's last activity, and then delete the documents (just the documents, not the account, just in case) a month after that.
I don't think you should worry too much about this. Someone who abandons a free service for an inordinate amount of time shouldn't expect to see their data. Especially when the cost of storing such data isn't nominal.
Send them one or two reminders and you should be fine.