It all depends on how much yeast you start with, how viable it is, and what temperature you can ferment it at.

If you plan to make a starter for a Wyeast pack, and assuming it's reasonably fresh, you can make 1.2 - 2 litres of wort at a gravity of 1.030 - 1.040. To do this boil 2 litres of water with 200g of DME (this 10:1 ratio can be scaled for any sized starter).

If you have a stir plate, allow the starter to grow for 24 - 48 hours (assuming the ambient temp is around 20C). If you don't have a stir plate, give the starter a good shake every time you pass it. If the ambient temp is about 20C, two days is fine. If it's less than 18C you might need a third day.

If you're growing a starter from a slant, start with a small initial starter (about 250ml) for 24 hours and then step this up to 1.5 - 2 litres for another 48 hours.

I normally turn off the stir plate the morning of the brew day and allow the starter to settle. I then decant off most of the liquid and swish the yeast cake around in the remaining liquid before pitching.

The

Mr Malty pitching calculator is a great resource for working out how big a starter you need.