One rule is to take the OG, subtract 1, and divide by 8. So for a 1.048 OG Pils, you should lager for 6 weeks. For a 1.080 doppelbock, 10 weeks.

The really old rule of thumb would be a week for each degree Plato. So divide the OG-1 by 4. The Pils goes to 12 weeks and the Doppelbock goes to 20 weeks.

Kai may have some good input on this.