Depending on your car/engine, it works it out as 10000miles or 12months, whichever comes first I think.
On a MkIV Golf you reset it like this:
1. Push and hold the odometer button.
2. Turn on ignition.
3. Turn digital clock button clockwise, and choose the service you want to reset.
4. Turn off ignition.
Your Service indicator should now have been reset.
I would imagine the above is the same on a Polo. If not, hopefully someone will come along to help!