By default the Time Machine in Leopard backs up files that have changed in the last hour. If you want to change the interval from an hour, there is no direct option that allows you to do this. But if you are comfortable using the terminal, you can change the backup interval for Time Machine. Just get a terminal window open and enter the following:

sudo defaults write /System/Library/LaunchDaemons/com.apple.backupd-auto StartInterval -int 1800

You'll be asked to enter the admin password for your machine.

The last enter in the line above, 1800, is the backup interval in seconds. 1800 seconds is every 30 minutes. You can increase or decrease this time as you like. TO get the default value of one hour back, use 3600.