Howlong does it take to get to each collosi
This is difficult to answer. I'd say the time needed to get to the next collossus isn't that much, between 5 and 20 minutes, I guess. The actual time spent before you go to the next collossus may be much more, because of the answer to your second question:
and is there anything to do in between going to the collosi and actually fighting one?
Yes. Enjoy the scenery and the soundtrack. Sounds strange, I know, but you'll actually be doing that. Riding through the world of SotC is incredibly soothing, beautiful, and ultimately, fun.
Oh, and you can kill lizards, or try to shoot birds on horseback.
You will also find yourself seeing spots in the landscape and trying to get there, just to see if you can.
Does this game have a lot of replay value?
Also difficult to answer. I don't own a copy, which I regret. When I beat it, I beat it a second time right therafter, but then I had enough for a while, and gave the copy back (I borrowed it). That was two years ago. During these years, I wanted to go back frequently. It's not a game you spend three months with, it's a game you want to return to every three months. Plus, there's a challenge/time trial mode once you beat it, and there's an active Speedrun-community.
It's definatly worth its price.