a few weeks ago windows update deletes all my files from laptop
How often do you recommend backing up qt wallet? is one time enough?
Do it at least once in a month,
Pratically every time you make changes, like receive coins, or windows updates or changes, you would better do a backup , just to be safe
I think 1 time is enough. When you have transaction, it will be recorded in the blockchain so the current status of your wallet can be restored wilth all transactions history so far after the sync has finished.