Or if he uses it in a sentence, it might be "we will stay here for weeks and days". I noticed it when I told him we were going on a trip (speaking of a weekend trip) and he asked "For weeks and days?"
I've also heard him tell his brother, after correcting Jackson's misconception that perhaps he would someday heal from his Celiac disease, "No, I will have it for weeks and days".
It almost sounds biblical in its simplicity yet captures the brevity of a lifespan. :)
And then he turns back into a 4 year old, more interested in squirrels than lofty ponderings on eternity.