The earliest you should use it is when it has a good visible krasen on it. This indicates most of the yeast reproduction is complete and it is entering the fermentation stage. Depending on temps, and type of yeast, and if you are stirring or shaking the starter this can be anywhere from 12 to 24 hours. A 1/4 tsp of yeast nutrient helps biuld a healthy colony as well. I have used starters at 20-22 hours before with good results, but it depends on where the yeast are at. When you pitch near peak krausen it can really get going quickly in the primary. Sometimes in just a few hours.
The yeast stopped showing up in the pics right after the boil. You waited for the wort to cool to below 80 deg F right? Remember to follow good sanitizing procedures with the starter just as if it is beer once you've cooled the starter. If you don't have a stir plate, give the starter a real good swirl as often as you remember too. The extra oxygen helps in the reporduction stage.
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