I've heard of a few guys that milk seasonally. Have all the cows calve in a tight window and dry them all up the same time. Gives you a couple months w/o milking cows. I am not certain but I've been told that if the dairy is shut down (even for just a couple months) it must be reinspected before it can be fired back up. Not sure if that's true or not. If by part time you mean a few hours a day, I think you could do it but it would be extremely tough. It would seem that milking and feeding would take up the bulk of the time and the rest of the day would be "free" for other things. This is not the case. There is all sorts of things going on throughout the day to keep the operation up to speed. Hauling manure, repairs, cleaning, etc etc all get squished between milkings. In conclusion, part time dairying may be possible but if you want to do it right, pting won't work IMO.