It depends on the program.
In the case of his prime-time shows and his late-1960’s show, it’s an hour out of an hour.
In the case of his Westinghouse shows, 16 of the 17 editions are around 90 minutes long. In the case of the one exception, it’s the second half of the program.
Two items must be stated, however.
Firstly, it is only an assumption that all that survives is the material at UCLA. There could be more elsewhere.
Second, I am not sure of how many episodes were made of either of his 1960’s talk shows, and the way they are listed is not helpful in that regard.