How long on the stir plate

So I have pitched a 5 liters of 1035 wort starter with only 5.7 billion cells, which tells me I have only 1.1 million cells/ml.According Yeast Calc it will give me about 102 billions at the finish of the yeast job.Then I will need to do another 1 lt starter to get finally about 200 billion cells.
The point is I have put this starter on the stir plate 2 days ago, it has a nice vortex with a tiny foam in it, but it seems wort does not turn lighter in its color,thus I think its gravity hasn´t changed very much so far.Have you got any idea about how long should i wait to get this starter finished?

TIA !

Cheers

Two days should be long enough. I would remove it from the stir plate, and let the yeast settle. If you don't see any yeast on the bottom of the container. Take a sample, and check the gravity. If it remains at 1.035, you probally started with dead yeast cells.

Yeast take about 2-4 hours to double in count, and they will max out in concentration around 200 million per mL of well aerated wort. So Im sure you can do the math. You should have about 100 billion with .5L of starter and 200 billion with 1L And I know I might get some flak from someone but next time just make your starter as big as you need it to be the first time, unless its to big for your flask. The yeast will hack it and do their job just the same.

Easiest way to see if you've had fermentation in the starter is by shaking it.  If it foams up, it's at least started and is probably done fermentation.  This way you don't have to check gravity and risk contamination.  This works on any size starter, from 100ml on up.