Generally speaking, I think it'll go on for as long as WWE can maintain Mark Henry as this super dominant heel that's capable of beating anybody. People are responding to Mark Henry, they're just flat out liking what he's doing and it shows in viewership and crowd response during his matches. That's never a bad thing, that's what you want to have happen.
Since Mark Henry became World Heavyweight Champion, SmackDown! has been drawing in the 3.1 to 3.2 million range on average in viewers each week. That's up 300,000 to 400,000 viewers compared to what they were doing before he won the title. I'm not saying that Mark Henry is 100% entirely responsible for the big numbers SD! has been bringing in, but I do believe he's a huge factor. Mark Henry is a draw. Mark Henry is money. Damn...I'd have laughed in your face until I pissed myself if you'd have told me I'd be saying that 6 months ago.
As we all know, the WWE is tossing around ideas for WM and the ideas will be changed front to back probably a hundred times or more before the event itself. But, there's been talk of Mark Henry possibly holding the title to WrestleMania. If Henry continues to thrive as he has been and the blue brand continues to draw these larger numbers, then it won't surprise me. At the same time though, I agree that TLC or the Royal Rumble do seem more realistic for him to drop the title. Then again, look at how wrong we've been about Mark Henry and how he's been underestimated. If he continues to produce, then WWE would be foolish to take the strap off of him for a while.
As to who would do it, the likliest candidates are Sheamus and Randy Orton. Randy Orton has had several matches against Henry, two of them at big ppvs that he's lost cleanly, and I know he's won a few of them via disqualification. I think Orton is out of the picture for the time being. There'll be at least one or two more matches between Henry & The Big Show. I'd say that Sheamus is a good possible candidate as they've definitely been building him up over the past several months. He & Sheamus also have a history that can be used to easily lay the groundwork for a feud over the title. They've had a couple of matches, both of them pretty good, and each has one win over the other via count out.
It's kind of up in the air at this point. I know some are worried about Daniel Bryan being wasted and his losing streak and all that. There's still lots of time to build him into a credible opponent. It won't be nearly as hard as some think. All he'd have to do to have that moment would be to for him to be booked as dominantly attacking and taking out someone like Sheamus or Orton, or turning heel and just go on a tear through the SD! locker room. A few weeks of very strong booking and his losing streak will be all but forgotten.