The quests are generated every week with a random reward. Below is the list of things worth to consider before embarking of the adventure. I had 'excess unlocks', so I got a head start on the progression to the next Boss level. After some weeks passed and all the regions are unlocked, one may find it troublesome to navigate through the increasing number of available expeditions.

So, the last bullet point explains my situation, I think. at 50% progression to unlock apprentice hag, a medium mission will unlock the boss AND progress to unlocking the apprentice Brigand Pounder by 25%)

Excess unlock progression from expeditions which unlock the boss DO appear to carry over to the next boss.

For Champion bosses, successful short expeditions will increase the unlocking progress by 25?%, medium by 30?%, long by 40?%.

For Veteran bosses, successful short expeditions will increase the unlocking progress by 33%, medium by 50%, long by 67%.

For Apprentice bosses, successful short expeditions will increase the unlocking progress by 50%, medium by 75%, long by 100%.

Each expedition completed in a certain location will advance the search for a boss, represented as a progress bar under the location's name: once the progress bar is filled, the dungeon levels up, increasing the amount and variety of quests offered each week, and unlocking a boss expedition. To unlock a boss mission, a dungeon has to be explored and traversed several times. Found the following in the Darkest Dungeon WIKI :

Darkest dungeon wiki ruins medium