So yes, essentially, I would like a reality check.
Assuming an individual with very little experience using Bethesda's toolsets, very little knowledge of scripting, and a lot of free time on their hands were to turn their attention to modding Skyrim, realistically, how long do you expect it would take before they could handle building the equivalent of Skyrim's main questline? The scenario doesn't require the creation of new assets.
I ask because I'm sitting here making notes and churning ideas, but quite a few of them seem very daunting to someone who's not modded before.
A quest of that size (keep in mind I have not completed it, not by far, but I have a decent estimate of it's length)? A long time. Depending on how quickly you take to the CS and how easily you grasp whatever system is in place for quest design now. I can only speak from experience with the GECK but I spent two years or so and only pushed out a small amount of very simple mods, though I didn't devote all my time to it. (they're in my signature for anyone who still plays Fallout 3/New vegas...which is nobody now that Skyrim is here, but I might as well plug it)
Anyway, reality check: It will take a long time, a lot of work and a ton of patience and enthusiasm.
EDIT: If you really want to do this, here's how I would go about it. Start small. Make a short questline, work out the kinks, get feedback and learn from it. Then make a sequel, maybe make that a little longer or a little more complex, challenge yourself a little and again, learn from it. Then you go from there. Instead of making one massive quest, make a series of independant but connected questlines.
That way you deliver the content iin bite siezed chunks, making it much easier for yourself and allowing you to learn as you go and get community feedback and help. Once you reach the end there's no reason you can't just pack it all together into one mod and have the final version be the complete quest line.
If you want an example of this and own Fallout New Vegas. Check out the "New Vegas Bounties" mods. Last I checked there were two parts, but that was a while ago and I haven't been paying attention. Anyway, those two (or more) mods have their own quests that can be played independantly, but are connected so that when you play them in order you get the complete experience. They're good examples of how to do a series of quest mods instead of one giant quest mod. You can also see the creator improving and learning when you compare part 1 and part 2. The quests became more complex, there were more voice actors and overall there was a clear improvement.
I think that approach is the best one if you want to do a big story arc like you're describing.