The self guided tour will take approx 3 to 4 hours.
There are several events going on which may add up on time , you can check for ongoing events with them beforehand. At the moment "Behind the Seams" is on which talks about the costumes and how they were designed for all the characters. Several events were planned but th studio is closed as of now due to Covid-19.
Of course.Half day is more than enough and if you are going in summers the sun is out till 8pm in the night so even if you leave at 6pm there is enough time to come back to London(assuming that is your base)