In his prime, Taker could have definitely gone for 60 minutes. If he can work 30 minute matches now, why couldn't he work 60 minute matches in say 1997? Whoever says that Taker can't go for more than 20 or 30 minutes is considering Taker as he is right now and not as he was in the 90s. Do you remember that the first Hell in a Cell with Shawn Michaels was also over 30 minutes long and that when Taker signalled the tombstone and Kane debuted during that match, Taker didn't look half as tired as he looks now after such long matches?
And I have to agree with most of the people here who say that Shawn Michaels would be the best opponent in an iron man match. Hell yes, can you think of anyone else? It has to be either him or Bret Hart or Kurt Angle, but I still think that from an entertainment value, you will like the match more if it has Shawn Michaels in it because he has the right mix of skill/ technical ability/ charisma/ gimmick/ entertainment/ emotion that can carry the match for that long.
I regret that we were not able to see Taker vs Mankind, Taker vs Bret Hart, Taker vs Austin, Taker vs Lesnar and Taker vs Angle at Mania despite the fact that Taker had long and intense feuds with them, and also had great matches with them at other PPVs. But instead of them, we got to see Giant Gonzales, King Kong Bundy, A-Train, Big SLow, Mark Henry.... please, let Taker's last Mania be against a skilled guy like Alberto del Rio. I wouldn't fancy a third Taker vs HHH match since the third wouldn't be able to match the first or even the second.