First, delfinidae, welcome to Discuss Cooking.
As for the "stirring" terms, they are a lot like "small, medium, large."
I have no idea as to the "minutes" involved, but stir "occasionally" means just that. Occasionally. Not hovering over the pot/container. Just enough to keep things moving and from sticking.
Stirring "frequently" is, as the word suggests, to stir quite a bit more than occasionally.
Then, to stir "constantly" is to be present at the vessel and stir, stir, stir.
Again, I have no minute-by-minute advice as to the time necessary for each procedure. That would depend on what one is "stirring." Custards are different from gravy/sauce.
Best wishes on your quest and I hope you choose to stay with us at Discuss Cooking.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!