For the iOS app I can say that 18 months is more than enough for our current feature set and upcoming plans.
Even if we began displaying graphs of page views over time… I can’t see any need to go back more than a few weeks or months.
For historical data the idea of degrading is an interesting one. I think that the daily data becomes much less important as you go back in time. Even if we only kept daily data for 6 months, that would be enough for our use cases.
This is probably true for Android as well, since we have pretty similar UI, but I’ll let them chime in to be sure.
Let me know if you want to know any further info.