No need to use a macro, there is a built in feature called AutoSave. Read about it in the help file for more info, if you need. Here's the 2007 instructions:
Excel
Click the Microsoft Office Button , and then click Excel Options.
Click Save.
Select the Save AutoRecover information every x minutes check box.
In the minutes list, specify how often you want the program to save your data and the program state.
Tip The amount of new information that the recovered file contains depends on how frequently a Microsoft Office program saves the recovery file. For example, if the recovery file is saved only every 15 minutes, your recovered file won't contain your last 14 minutes of work before the power failure or other problem occurred.
Optionally, you can change the location (specified in the AutoRecover file location box) where the program automatically saves a version of files you work on.