It uses 10% of my processor for a minute before each chime, and randomly at other times.
It's also a waste of money. With a little more time on my hands, I just installed a simple mp3 player and use crontab (built in on Macs) to chime every 15 minutes between 9AM and 5PM. This is much easier, faster, and cheaper if you can google crontab and are willing to install mpg123 or something similar.
Here's my crontab:
0,15,30,45 9-16 * * 1-5 /opt/local/bin/mpg123 /Users/[YourNameHere]/Library/PreferencePanes/Cuckoo.prefPane/Contents/Resources/Sounds/Pure\ Cuckoo.mp3