Question…would an 18 hr cold retard be too long for a 100% whole wheat?
(Rouge De Bordeaux flour)
Short answer: no.
The longer answer depends on answers to a couple of questions:
I am presuming you are talking about a fermenting dough, as, otherwise, that's just a cold autolyse, which is fine, too, but, as per Benny and Ilya's input in a previous topic, if you are referring to a cold autolyse, it's safer to make it a saltolyse, which is simply where you add salt to the flour/water mixture. 18 hours at fridge temp is much more than enough of time to achieve all the possible benefits of an autolyse so the added salt slowing it down will be a good 'safety net' without there being any risk that it won't be autolysesd or softenened 'enough'.
When undertaken with fermenting dough, I tend to think of the retard not really as a separate step but as modifying a portion of an existing step - e.g. it's not: bulk ferment then retard; it's more: retard as the final part of a bulk stage.
Unless you really can get your dough down to 1C or so very quickly, of course, (e.g. in a blast chiller,) in which case it really can be a separate step and can start when your dough has finished the previous stage in your process.
Welcome to TFL !
I would also suppose it has to do with how much levain or yeast is in it.
My typical miche is 90% WW, with 3.5% pre-fermented flour. (roughly 7% levain of 100% hydration.)
A 4 hour room temp bulk ferment, then shaped and overnight fridge proof at 41 F, is just about right for my formula. 5% PFF over-proofs it at those timings.