James at Grognardia wrote a post called Abandoned which touches on a point I realized that I haven't talked about before. How does a sandbox campaign end?
My experience mirrors my own to some extent. Many of the campaigns I ran just peter out. I think only one or two stopped mid-adventure.
I had only one campaign that consisted of single story arc that just naturally ended with everybody agreeing that the character achieved their goals and really had no reason to adventure. That involved two players; Tim of Gothridge Manor, and Dwayne of Gamer's Closet. Tim played a blacksmith and Dwayne a Black Lotus, the Overlord's secret police. They were given an initial mission to discover what the Duke of Bernost was up to as the Overlord suspected treason. What ensued was a campaign lasting several months where the players uncovered the fact that the Duke was planning a rebellion and that his secret was the use of cannons powered by Dragon Powder (gunpowder). Part spy vs spy and part technical mystery (I used the details of manufacturing gunpowder as a plot element) the mission was the campaign. At it's conclusion the characters successfully returned to City-State at which point everybody agreed that with their rewards and with what happened there was no reason to adventure.
I also refereed a single campaign over five years that had three major halts. It involved Tim and Dwayne again along with other players from time to time. Dwayne played William Enderil a merchant, ship captain, and a reaver of the sea. Tim played Draco-lindus, Captain of Enderil's forces. The first phase ended with the Enderil's ship being wrecked on a beach, the second phase was played with Tim solo as a independent Mercenary Captain in City-State. The third phase was Dwayne joining again and the two winning fame and fortune by conquering a kingdom and Draco-Lindus becoming a Duke of City-State and Enderil a Baron and a powerful merchant prince.
But James is accurate in that it very hard to restart a campaign after a major halt. It isn't the lack of information so much but rather the players losing touch with the feel of their characters. It like the old saying that when you return to your hometown after a long absence it often feels like you can never truly return home.
The vast majority of my campaigns I managed to manipulate the situation so that the players could think of their characters continue to live their lives without adventure in the situation where the campaign stopped.
Part the reason I can do this because I use the World in Motion technique so heavily. So the character lives w like people's live do in reality with quiet times between the adventures. End the campaign on the quiet time it doesn't feel so jarring.
As part of managing the campaign as a World in Motion the players generally have goals for their characters. The initial ones that are generated as part of my pre-game and the later ones that come out of playing. If enough of them have been fulfilled then the campaign can be ended gracefully.
Finally you can have a plot in a sandbox campaign. It specified in the future timeline of events you created at the start not a story. Unlike a story it changes as the characters interact with the locales and NPCs. Well designed plots can't be resolved with a simplistic solution, while the course of the campaign is determined solely by the players choices eventually the players will do what is needed to resolve the heart of the plot. Once accomplished and the intendant awards given that is often a natural stopping point for a sandbox campaign.
For example in my weekly sandbox campaign with Tim, Dwayne, and Rusty BattleAxe I been dropping hints, portents, and visions of a threat involving someone or something with the symbol of a falcon. Until recently most of them landed with all the excitement of a wet papertowel. It not a criticism as each of the players had their hands full with collective and individual goals.
The timeline of events involving the falcon symbol marched pretty much uninterrupted for the past year of campaign play. But then to a combination of events that occurred, and yet another dropped clue, now the players are greatly interested.
Most of what happened before landed in the lap of Rusty Battleaxe Thothian Mage Syrivald. And I deliberately made it a mess of vague vision and unclear clues so it was understandable why other goals were of more immediate interest to Syrivald. But then Ashling, Tim's Elven Mountebank, was killed by a shadow dragon in the Black Taigh.
One Ashling's motivations is that he felt the his people, the Elves, have stagnated. That they sit behind their guardian Taighs walling themselves off from the world while humans and other race spread everywhere. Throughout the campaign Ashling has placed himself in the thick of things but the driving goals have been Syrivald's, researching old magical circles, or Eoleandar's killing the last of the full-blooded Viridians.
When he died he found himself in the Blessed Realm of Silvanus and there in a long conversation with the god Silvanus himself Ashling realized that he been focused on the past. The Viridians, the old circle of magic. That the falcon represents the threat of the future and the true challenge that he and the rest of the party were to face.
Now the weather vane of the campaign has firmly swung south to Tiethoir and the unraveling of what the falcon means and what threat does it represent. The campaign may continue after it's resolution but I suspect this will be the end game as the players are past name level and threat they face is going to be one the biggest challenges they have faced yet. And it has nearly claimed one casualty already due to a teleport spell going awry and low.