What I often find confusing is the notion that those of us who make frequent use of fast-travel mechanics in these sort of games are "missing out," or that those who completely eschew the use of it at all are in some way going to experience "more" of the game.
Now, I actually got bored with Oblivion before I could finish it - but I've completed all of both Fallout 3 and Fallout: New Vegas, which use the same manner of fast-travel. I've got loads of playtime in all three of those games. I've seen every inch of those maps. I've explored every dungeon, found every easter egg, literally experienced everything that game has to offer me. I also used a lot of fast-travelling. Someone who's played all through Fallout 3 without any use of fast-travel hasn't had any more fun than I did, they weren't any more immersed in the world than I was, they never saw anything that I haven't seen as well.
My method of exploring a game environment is that the first few times out from a new sort of "hub" area (or base of operations, etc) I'll do on foot. That way I can go off the beaten path, see if any interesting places crop up, etc. But once I've been through an area two or three times and know it has nothing to offer me other than maybe another random encounter (which I've already seen a few times by then, as well,) I'll use fast-travel.
For me, nothing breaks my immersion more than being bored or feeling I'm doing something tedious. When I want to just go on a stroll and feel "immersed" in the world - I'll do that. But I also like the option of knowing that if I'm getting bored of walking down the same road for the umpteenth time - that I have a way of skipping through the tedium and getting to something interesting.
Obviously - that's just me. As far as I'm concerned, the "cost" of fast-travelling is already innate. There shouldn't be any other (totally arbitrary) "charge" for doing something that's only keeping a game from becoming boring.
Many people seem to think, "It takes too long" but that's simply just a foolish thing to say. In Oblivion, I ran from Leyawin to Anvil (the furthest possible distance) in just 22 minutes ... However it was only 10 minutes on a black horse. In fact, horseback is my prefered method of travel because it's faster than running but not as cheap as fast travelling. To me fast travelling is something that spoils the experience and I've always been passionate about the subject. My friends never understood, so I'm curious ...
Yeah...
I have a 4-month old kid at home. That 22 minutes? That's often my
entire play time for a day. :b
Ten minutes is too long if it's something you don't want to be doing in the game.
Heck, one minute is too long if it's something you'd really rather not do, and isn't really intrinsic to the game...