Premixed Bircher muesli (cereal) - soak overnight or ready-to-eat?
Both.

It’s just a matter of taste and convenience what you prefer. 


The long-soak gives an overall softer, mellower result and requires a minimum of planning. The overnight-soaked version may be more digestible, but this largely personal, not necessarily a general rule. It’s also close to the “original” version.
The pour-and-eat method preserves the flavors of the individual ingredients and retains the “crunch” of some of the components. It’s also more convenient for packed meals, if the liquid component is added at a later time - not refrigeration necessary (except for the milk or yogurt, obviously).
You may even try out muesli cooked by a porridge method, but of course you’d be going against the philosophy of the muesli inventor...


The “original” Müesli popularized 1 by M. Bircher-Benner uses rolled oats soaked for 12 hours served with freshly grated apples and nuts. Modern mixes are typically based on a mix of grains, nuts and dried fruit. If a mix contains lots of “gets-soggy-quickly” ingredients like corn flakes or puffed grains, often combined with finer oats, it’s usually an indication that it was intended to be eaten immediately after adding liquid, but there is no absolute rule, your tastes matter.



1 But not invented. Raw oat- or wheat-based preparations have been a staple in many cuisines and Bircher-Benner reported that he learned about the dish in the alps.