Milking event is a new dialogue that will appear in 'talk' option with Roghak after you gain 15 likes or more with him. It's not a quest and you can repeat it daily.
I now but it doesn't show, I even start a new game currently at day 188? all main missions finish except for Scraps but I don't think that would make a difference since his story for now is isolated.
If the Medicine quest has been completed at least once, and you have 15+ like, try clicking on Wolbauk's Pen, clicking Leave, and repeating until it appears. Took me about eight tries.
