The data is being stored a once a minute resolution, and on our server, so not eating up valuable disk space on your Pi. Right now there are no limits on it at all (size or history wise), but this may change in the future. We'd announce it ahead of time though so don't stress over it disappearing.
As far as the history, I'm seeing history back before today on my sensors, and it should go back much further. Maybe this is a specific issue, can you PM me and let me know about which sensor (or is this all of yours), and provide a screenshot so I can investigate?