Here's what WhiteLabs has to say about it:
White Labs Pitchable yeast is packaged with 70 to 140 billion yeast cells, which corresponds approximately to a 1-2 liter size starter. Lag times are typically between 12-24 hours for a normal strength brew.
A yeast starter is a small volume of wort that you add to your yeast to initiate cell activity or to increase the cell count before using it to make your beer. The yeast will grow in this smaller volume, usually for 1-2 days, which then can be added to 5 gallons of wort.
While a starter is not always necessary, White Labs recommends making a starter if the Original Gravity is over 1.060, if the yeast is past its "Best Before" date, if you are pitching lager yeast at temperatures below 65F, or if a faster start is desired.
In a medium sauce pan, add 2 pints of water and 1/2 cup Dried Malt Extract (DME). Mix well and boil the solution for about 10 minutes to sterilize. Cover and cool the pan to room temperature in an ice bath. This will give you a wort of approximately 1.040 OG. Keeping the Original Gravity low is important because you want to keep the yeast in its growth phase, rather than its fermentation phase. The fermentation phase will create alcohol which can be toxic to yeast in high concentrations.
Pour the wort into a sanitized glass container (flask, growler, etc.) and pitch the vial of yeast. Cover the top of the container with a sanitized piece of aluminum foil so that it is flush with the container, but will still allow CO2 to escape. Vigorously shake or swirl the container to get as much oxygen dissolved in the solution as possible. Allow the starter to sit at room temperature for 12-24 hours, occasionally shaking it to keep the solution aerated.
You probably won’t see any visible activity, but the yeast is busy taking up the oxygen and sugars in the solution and growing new cells. After the yeast has consumed all of the nutrients and oxygen, it will form a milky white layer on the bottom of the container. If you are not planning on pitching the yeast right away, you can store it in the refrigerator with the foil still in place. When you are ready to brew, decant off most of the clear liquid from the top, being careful not to disturb the yeast layer below. Once the yeast and your wort are at approximately the same (room) temperature, rouse the starter yeast into suspension and pitch the entire quantity into your fermenter.
Typical Starter Volumes for 5 gallons:
To activate the yeast: 1 pint (with 1/4 cup DME)
To revitalize yeast past its Best Before Date: 2 pints (with 1/2 cup DME)
To brew a high gravity beer: 2 pints (with 1/2 cup DME)
To brew a lager beer, starting fermentation 50-55F: 4 pints (with 1 cup DME)