It depends on the campaign, the PCs, and DM's whim.
Typically, if I'm running homebrew, it is a unique campaign for a set of players and PCs (though the exact roster of both can change).
If I'm running storebought, it depends upon whether I want continuity. I've run several campaigns in Greyhawk- 3 of which were interrelated (low, medium and high level PCs in the same timeline), ditto Forgotten Realms (including the 3 interrelated story arcs).
However, most of the time when I run a storebought campaign, I'll pick a time point and start the campaign, each like a little alternative dimension.*
*And occasionally, I've had PCs cross into those other dimensions- like the time a particular True Neutral Assassin met his Lawful Good Undead Paladin counterpart guarding a holy site in this alternative world...
Typically, if I'm running homebrew, it is a unique campaign for a set of players and PCs (though the exact roster of both can change).
If I'm running storebought, it depends upon whether I want continuity. I've run several campaigns in Greyhawk- 3 of which were interrelated (low, medium and high level PCs in the same timeline), ditto Forgotten Realms (including the 3 interrelated story arcs).
However, most of the time when I run a storebought campaign, I'll pick a time point and start the campaign, each like a little alternative dimension.*
*And occasionally, I've had PCs cross into those other dimensions- like the time a particular True Neutral Assassin met his Lawful Good Undead Paladin counterpart guarding a holy site in this alternative world...