Cindy's Answer

Good is a duration marker as you stated, to give duration as per the article on Marker Training.

If I say SIT then they are expected to continue sitting until I release with YES or move on to another behavior.

I use GOOD to reinforce their holding the position while they're learning. GOOD is also used during moving exercises like heeling so I don't want it to mean STAY, it simply means "keep doing what you're doing".