Yeast starter from commercial beer yeast

I am planning on using some yeast from some sweetwater ipa.

I have about a 1.5L starter going on my stir plate now. 2 questions.

-I used the yeast from 4 bottles, and I don't care to do a cell count. Anyone know about how much there is in a bottle? I'm guessing 4 bottles is still less than say a smack pack. My IPA is 1.070 so I figure a 1.5L/decant/2L should produce enough.

-I understand it may take a little longer to get going. 36 hours before chilling and decanting good, or should I go 48?

I would make a starter of only a pint (or 500ml) with the dregs from the four bottles first. Then after a couple of days add a liter of wort, see how it's doing and then maybe step up again. The yeast in the bottles has had a pretty rough time and there is probably a very small amount of actual viable yeast in the bottles, especially from an IPA. It can be done, but you'll have to make sure you are very sanitary and patient.