Well, it could be anywhere within that range, given the time that I put into each of the previous four TES Games:

• TES I: Arena <100 hours. Admittedly, I've never beaten it. And I also only really tried it out once it became free-to-download back in 2004. I don't think I've touched it in a couple years.
  
• TES II: Daggerfall - >2,000 hours. This was my first TES game, and still stands to me as the downright best. Most TES fans that've been around before the 21st century will agree with me that BOTH MW and OB failed to match up. This is the only TES game I still play regularly.
  
• TES III: Morrowind - ~1,200 hours. The game was a bit of a let-down for me compared to DF, but still it was good nonetheless. Plus, its phenomenal level of mod support (exceeding that of OB) has contributed to its longevity. I occasionally play this game.
  
• TES IV: Oblivion - ~900 hours, if I remember my save times all right. To me, this was a disappointment; still a fun game, but that's because it was being compared to games of its OWN time, and overall... I feel that the quality of games has degraded significantly over the past 10-15 years. It fixed a number of things that badly needed it in MW, but also broke new things, (the PC interface, anyone? Or the inability to delete spells?) cut out a lot of features during development that shouldn't have been, and also, in the madness, neglected to fix a bunch of simple problems that were still in MW. (rain still falling through bridges and roofs?)

It's a bit early to truly tell how Skyrim will come out. Basically, if it turns out to be an "Oblivion +1," I'll likely play it less than I did OB; chances are in that case few PC gamers will bother working on mods for the game either, and instead focus on the otherwise-as-capable Oblivion. If the game lives up to all its promises, (unlikely but still possible, given the history gamesas has) then it could run into the quad-digits. Doubtful that it'd displace Daggerfall.