As has already been mentioned, this is answered on pg 180 of the PHB.
It's "No Action" - you get one roll, you either know it or you don't.
I allow players to roll whenever a creature is mentioned, or encountered.
So, if the oplayers are hired to clean out a kobold lair, they would roll immediately to find out how much they know about kobolds.
Then in that lair, when they encounter a Guard Drake, they roll to see if they know what it is and and how much they know about it.