Is it possible my yeast starter finished in 15 hrs (or is close to finished)?? I made it yesterday afternoon and just checked it... very little activity w/ thick yeast slurry on bottom. I pitches a wyeast pack into 2.5L of wort and fermented at 69F.
Very likely. When are you planning on brewing?
You can cold crash the starter, decant the liquid away and add fresh wort to keep building it up.