i usually let the cereal soften a bit before eating it; not to the point where it’s soggy and/or mushy but just long enough so that the milk will retain some of the cereal flavour. :’)
I’m in the bizarre “neither-I eat cereal without milk” camp. I’d probably go with immediately because if you let it soak it ruins both the cereal and the milk (though I still count it ruined as soon as it touches)