For more than three hours at that. Have a look here - he starts to comment at around 3:33:35. (For those of you who are fairly new to chess, Boris Spassky was the original post-war prodigy, making the Candidates as a teenager and setting the record for the youngest GM until Fischer broke it. He was also the world champion from 1969 to 1972. In case you think those were the dark ages, he has an even score against Kasparov, and that's not because he beat Garry in simuls when the latter was in short pants.)