This article from the BBC has great tips on using a slow cooker.

In particular, this is a very hand tip:

10. How long should I cook it for?

If a dish usually takes:

• 15 – 30 mins, cook it for 1 – 2 hours on High or 4 – 6 hours on Low
• 30 mins – 1 hour, cook it for 2 – 3 hours on High or 5 – 7 hours on Low
• 1 – 2 hours, cook it for 3 – 4 hours on High or 6 – 8 hours on Low
• 2 – 4 hours, cook it for 4 – 6 hours on High or 8 – 12 hours on Low

Root vegetables can take longer than meat and other vegetables so put these near the heat source, at the bottom of the pot.