How often should I back up my wallet if I use Bitcoin-Qt client? I am new to bitcoin and the first client I installed was Bitcoin-Qt (Bitcoin Core v0.9.3.0-gcea5e49-beta). 

After installing and downloading the blockchain I immediately created a backup file. The other day I got my first bitcoins. Should I create a new backup file every time my wallet is updated (get more coins/spend coins) or its unnecessary (my first backup file contains all info needed for recovering all transaction history for my private key)?
If you use default settings you have to backup your wallet.dat after you unlock each 100 your addresses. You also must backup your wallet.dat after you set a passphase on it. Do not create backups by copying your file while your client is running. Eigher create backup with a client menu or shutdown client and copy file to a different folder.