You can actually ask your Mac to remind you every hour, every half an hour or every 15 minutes by using clock feature, announcing time.

Clock Preferences Pane

First, go to System Preferences. Click on Date & Time to go inside Date & Time Preferences Pane. Inside here, you will see three tabs, which is labelled from left to right Date & Time, Time Zone, and Clock; Click on Clock.

"Announce the time" Checkbox

After that, tick the checkbox labelled [Announce the time] and choose the most suitable option for you, either it's every an hour, every 30 minutes or every 15 minutes.

Customize Voice

If you don't like the default voice, which is the voice of Alex (I used automator to make Alex announce the context of "Think Different" ad inside this file), you can change it by clicking on Customize Voice.. button. Clicking on this will reveal a pop-up menu. You can customize the person who is in charge of speaking, the rate of voice and the volume.

Testing Voice

On the bottom-left corner of the Customize Voice.. menu, there is a Play button. It's there for testing purpose. In addition, here are some of testings dialog :

Alex
"Hi, I'm a new voice for Leopard." (Greetings!)

Junior
"My favorite food is pizza." (Who cares)

Agnes, Kathy, Vicky, Victoria
"Isn't it nice to have a computer that will talk to you?" (It may be nice)

Cellos
"DuDaDuDaDamDimDim.. DuDiDu.. DamDamDam.. DuDaDuDaDuDaDuDaDuDaDuDaDu..." (Yeah, DuDamDam~)