I couldn’t use my Tartine bread levine “on time”: is any way to quickly “refresh” levine or I’ll have to start all over and wait another 6 – 8 hrs?
Long its been since you fed it I typically go ahead and use a small amount.
Example:country loaf is 200G and a 6-8 hour leaven. If mine has gone past that mark, say 12 hours, I simply reduce the amount of leaven . Generally 15% or 150G on a country loaf formula.
For me, I have used it days later. I know he says to use a young levain,but I don't think I've had a bad result using a more mature one.
I don't think anything happens "quickly" in the world of sourdough. But also you don't have to throw the levain out and start again. I don't think mine has ever gone way too long, so, as David suggests, you can try it anyway.
But just to add here a reference to a previous discussion on multiple levain builds. I have tried this myself in the past as an experiment, but I haven't continued to use the method as it didn't seem to improve significantly on the standard Tartine regimen. There's a nice discussion on this thread.
I wanted a flavor more like the bread sold at Tartine Bakery which is much more sour than what the recipe in the Tartine Bread book creates (which is too mild for my taste). I build my levain from a starter that is has peaked and collapsed and then I let my levain get quite mature before using it. So to answer your question, yes, you can use the levain beyond the 6-8 hours, but I probably wouldn't let it go more than 16-18 hours at room temperature. If you refrigerate the levain, then YMMV.