The default time machine backup interval is every hour. Sometimes this can be really annoying so, if you find yourself in need to change this, here's how:

Navigate to /System/Library/LaunchDaemons and locate the file named com.apple.backupd-auto.plist.

Inside this file you will find the following content: <key>StartInterval</key> <integer>3600</integer> 3600 means hourly (60 minutes X 60 seconds). If you multiply by 4, for instance, and use that value (14400) you will have automatic backups every 4 hours instead. Or whatever interval you prefer. Happy backing up :)