How long to ferment for that nice zip for those who like a sour sourdough?

I started a very successful sourdough microbakery but my overnight ferment is just 12 hours which results in a mild flavor that pleases even those who dont like sourdough. My sour loving customers want more sour so I am offering that as an option for classic loaves. How long would you all recommend the shaped loaves ferment in the fridge? I have read that waiting too long affects rise and I won't want flat bread but more zip would be good.