My current OD&D campaign set in the Wilderlands of High Fantasy has been running almost every week since the end of September (See New Campaign: OD&D Set in the Wilderlands of High Fantasy). The group started in Thunderhold and was traveling to the City-State of the Invincible Overlord. I expected two to three sessions of travel, but the party has been side-tracked by small dungeons, bandits, THE Goblin Horde, a dragon, treasure maps, people to rescue, etc. They should finally reach the City-State today, some 16 or 17 sessions after I expected. Sandbox campaigns are truly unpredictable.
I say that they should reach the City-State today, but there is on major event that needs to happen first. They reluctantly entered into the service a of young (and very dumb) dragon as his "official treasure finders and sacrifice cooks." (See OD&D Wilderness Campaign: Hired by a Dragon at a "Generous" 15% of Treasure Found for more info.) They've been plotting to subdue their "boss" since February so they can sell him in the City-State and become really rich. Today they try. It's either going to be a success or there will be a lot of dead or maimed PCs. Given the dragon's low intelligence and youth, they actually stand a fairly good chance of success as they've worked out a good plan. I can't wait to see what happens.