only for one year, after one year of read only access, intuit deletes it completely
there is no user back up for QBO, so the best you can do is sign in as the accountant user and export the general journal to excel for audit purposes
QBO data handling capability for reporting is not adequate, if there is more than 5K records it times out and reports that you have too much data. You will have to do a journal report in date range segments, like Jan-Feb, Mar-Apr, etc etc. How many months you can span per report is a guessing game since the report can not deal with more than 5K transactions.