My understanding is also that insight into Dhamma is gradual; at least, that’s how I think it works at the Sotapanna stage. Sotapanna magga phala happens in a thought moment (within a citta vithi), but that moment is not noticeable. Months (or perhaps years) later, a person then realizes his life has really changed, that he is no longer capable of doing any acts that may bring rebirths in the apayā.
I used to know someone striving to quit smoking, and there maybe an analogy here. He followed a regimen such as substituting smoking with running, contemplating the bad consequences of smoking, etc. Then one day he realized that he had not smoked a cigarette for months, and he felt just fine. He couldn’t pinpoint a moment when the “giving up” of cigarettes happened.
Purifying our minds probably works the same way; it involves removing defilements (which are kind of like addictions). Over time, we may notice that certain defilements are no more, but we cannot pinpoint the magga phala thought moment that eradicates those defilements.
Interestingly, I’ve come to notice that many breath meditation practitioners (anapana to them) believe in such a “eureka” moment. They hold that when the mind becomes totally present (via the breath) then the “light bulb goes off”, bringing a flash of insight, and that’s how magga phala happens. Here, we have learned over and over that breath meditation is NOT anapana.