1 hour 34 minutes, 6 colossi dead!
Messed up a little on Colossus 4 (horse), he took me half an hour from temple to death, because I forgot how exactly I was supposed to beat him and I was being impatient.
But then I did amazingly on Colossus 5 (bird). I shot an arrow, he came off the tower, he swooped down, I missed, he swooped again, I caught, and completely killed him without falling off once. Only about 12 minutes from temple to temple.
Colossus 6 was easy, as I remembered. I think he took about the same amount of time as Bird, but I fell off him once.
They only really get challenging from the seventh onwards, and even that is easy. 13 is when they become hour-long battles (from my experience).
Also, it seems they used all of the best music in the beginning of the game. All of the music is great, but overall, the first few colossi have the best tracks.
I'm going to take a break from it now.