I know there are many variables but with a typical yeast starter created from a smack pac that starts with 100 billion - what tare we talking about - double?? triple???
What I gathered from a jamil podcast, doublish is the max for any kind of starter whether it be smack pack, dry etc. I could easily be wrong though, it's happened before.
You'll double your cell count, the length of time that this happens is dependent on whether the starter is continually agitated, shaken occasionally or a simple starter that is simply allowed to ferment.
If you haven't listened to the Brew Strong podcast on the topic of yeast starters, it's well worth it.