TARTINE LEVAIN

is used to build the final leaven. A 'ripe starter' usually refers to a culture that has completely run through its cycle after feeding (doubled or tripled, fallen completely, and smells of vinegar). Depending on your specific conditions the timing may vary. But following the books instructions and having a 100% hydration starter rising and falling predictably it should be quite ripe if feed a 20% inoculation and kept at 70-ish degrees in roughly 12 hrs. This is the case with my counter starter at least. This 'ripe starter' is then used to build a leaven which is ready for the final dough when it has risen by 20-30% (young leaven). This could take a varying amount of time depending on % of inoculation (ripe starter seed). If you use 5% of 'ripe starter' as is recommended in the formula, and keep it at 65F it should take around 4 to 8 hrs. But if you use 20% inoculation and keep it at 72F it will be much shorter (1.5 - 4 hrs.). My personal starter, if feed a 5% seed and kept at 65F, takes around 7-9 hours to reach the young leaven stage. But if I feed it a 20% seed and leave it at room temperature (74F) it reaches the young leaven stage in around 2 hours. I imagine, from the video you're referring to, that the latter is closer to what he is getting at.  Hope this helps.