Since it sounds like they are already talking about his next defense will be at night of champions, I'd figured I'd update with assumption he holds till then. This is only including one reign per wrestler, so for instance John Cena's 280 day reign is not included. The number is days held.
- Bruno Sammartino - 2803
- Bob Backlund - 2135
- Hulk Hogan - 1474
- Pedro Morales - 1027
- John Cena - 380
- Randy Savage - 371
- Diesel - 358
- CM Punk - 301+
- Billy Graham - 296
- The Ultimate Warrior - 293
- Yokozuna - 280
- John "Bradshaw" Layfield - 280
- Bret Hart - 248
- Shawn Michaels - 231
- Triple H - 210
- Randy Orton - 203
- Stone Cold Steve Austin - 175
- The Miz - 160
- Brock Lesner - 152
- The Undertaker - 133
- Eddie Guerrero - 133
- Kurt Angle - 126
- The Rock - 119
- Chris Jericho - 98
- Sheamus - 91
As a bonus, here's the number of days Punk would hold if he holds it for the next few PPVs
- Night of Champions - 301
- Hell in a Cell - 336
- Survivor Series - 364
- TLC: Tables, Ladders & Chairs - 392
- Royal Rumble - 434
And one final fact is Sheamus will have the third longest World Heavyweight champion reign ever by the time Night of Champions comes around. I don't know about anyone else but these back to back long reigns sure is getting boring. Yeah SCSA, The Rock, and Angle were all in the title picture a lot, but at least there wasn't this 6 month period where they never lost at all.