I don't believe you can 'go back' to issues older than when you initially subscribed. As far as going forward - magazines and newspapers are typically saved by B&N for 1 year or 10 issues, whichever comes first. Then they are automatically deleted from your account.
HOWEVER, if you archive a particular issue, it takes it out of this auto-delete queue and it will be saved forever (or, at least, as long as B&N exists), even if you then unarchive it later on to view it again. You can do this for every issue.