How long does a yeast starter take?

I have a first step yeast starter rumbling gently on my kitchen counter. I'm planning on doing a second step before I brew. I'm wondering how many hours I should generally alot for each step.

24 hrs? 36 hrs? more? The wort I used was at 1.04 OG. Bubbles have slowed to about 2/min after 36 hours. Are bubbles a good gauge?

I'm out of DME, so I won't start the second step till tonight when I get home tonight. Hoping to reach a good cell count by Saturday morning-ish.

JPK

I usually let mine sit at room temp until the obvious fermentation stops.  After about 24 hours on the stir plate, I slow down the stir bar until it just keeps the yeast from settling to the bottom (no obvious movement on surface of fluid) and look for foam build up as a sign of continuing activity.  In my experience, it takes most yeasts it will take 36 to 48 hours to fully ferment out.  Lager yeasts and some slow starters tend to up to 72 hours before finishing the majority of their work.