Approximately how long should we allow to your the castle?
The guided tour takes an hour, plus there is the museum, some walking around, waiting for the tour start - 2 hours should be minimum.
Outside of the castle along the west wall is a display of construction methods for the castle.
Also don't forget: you park at the bottom and have to WALK up and down. I'd plan at least 15 minutes each way extra for that, possibly more, depending on what shape you're in ;) Hour for the English tour, hour for other stuff like walking around the grounds, checking out the view, visiting the museum, gift shop, cafe, etc. 2.5 minimum IMO.