Create an app with Automator that shows a notification and play a sound (available here).
Put it in your /Applications folder.
Type this in a shell:
Then, add these lines to the crontab:
# Every hour at 30 from 10h to 18h, on Mon-Friday, remind me to take a break. 30 10,11,14,15,16,17,18 * * 1,2,3,4,5 open /Applications/TakeBreak.app
Save the crontab and check the result:
- "Display in Notification Center" (at least "1 recent item").
- "Play a sound for the notifications".
Disable the others.