Munich to King Ludwig's Castles - 1/2 day trip?

I don't think you could do it in half a day. It takes about 2.5 hours from Munich to Neuschwanstein. You'd have to be there early so you can make your ticket reservation time. The tour takes about an hour, then another 2.5 hours back to Munich. Also include time getting up the hill to the castle, waiting time, bathroom breaks, photos, etc. As you can see you are already looking at about 6-7 hours. Hohenschwangau will add another hour to the itinerary. You really need a full day to do the castles if you can. There are other castles and palaces closer to Munich like Nymphenburg. I hope this helps some.

The first reasonable train leaves Munich Hbf at 6:53 (there is one 2 hours earlier but it is bus part of the way and only arrives 20 minutes earlier) and arrives at Fuessen after one change at 8:55, after which you need a bus to the Neuschwanstein "castle", then get tickets, then the slog up the hill or pay for the horse drawn wagon, then wait for your tour. When the tour ends, do it all in reverse.

The first train not requiring a change is an hour later but only one minute faster.

So, round about 5 to 5:30 transportation time depending on bus connections plus 2 to 3 hours at and around the "castles". I say "castles" because they are actually 19th century follies, and were never intended to be castles despite the hollywood treatment.

Thank you very much for the detailed responses.
Very helpful.