As for your questions: I'm in a pretty temperate environment; not too hot, not too cold. I know it's that they're individually drinking for too long (in my opinion) and not different people always there. I was watching my super soldier very closely for nearly a year in game time, and he would stop and drink forever before I made him a flask. While watching him, I noticed some other notable people that never left either. I think they just spend way too much time drinking. I don't know why they're coded to spend this much time.
There's two sets of things going into all this. On the one hand, it sounds like you may have some logistics issues, most frequently with not enough available barrels and made worse by crowding. When a dwarf wants to drink, they path to an available barrel with booze; as I understand it, this barrel will then be considered "pending use" until they get there, drink from it (generally described as "upend barrel over mouth, chug until you've had a month's worth" as no tools or containers are yet used), put it down, and the game gets around to marking it as available again. In a crowded stockpile, dwarves may be constantly re-pathing around other dwarves or dropping to half speed ("crawling") to fit past each other, which may make the problem considerably worse. Additionally, barrels are not usable from when they get marked as pending transit from brewery to stockpile until when they arrive and are marked available. In some cases, thirsty dwarves nearer the brewery may claim recently-produced barrels of booze still in the brewery, which causes more dwarf congestion between drinkers, haulers, and brewers and prevents them being properly cleared out to reduce brewery clutter.
Some efficiency suggestions: Have your main booze stockpile be thin rather than just a square; one of the best arrangements is to have it be a square or few deep all around the edges of your dining area, so dwarves will naturally path to different parts of it without congestion. Have more barrels of booze than you think you need, and then some more than that; for smaller forts where efficiency is important having your main drinking stockpile with perhaps 20% more available barrels full of booze than you have dwarves is a good idea, with supplemental stockpiles near outlying areas (forges, farms, etc.) where dwarves spend a lot of time to decrease both pathing and travel time. Make sure the stockpiles are closer than the actual breweries to where your dwarves generally are and you have some spare haulers, to prevent congestion at the production source. Consider marking some high-traffic marked paths to optimize pathing near and in the stockpiles, and some low-traffic marked paths just outside the brewery to keep dwarves who don't have business there from getting in the way.
Looking at the other direction, "a dwarf will drink booze an average of four times per season", so that's 16 times a year, or about 23 days per. Consider how much time a typical late-medieval through pre-industrial working man would spend at a tavern; remember than in many societies, beer (etc.) was providing a significant fraction of your daily calorie intake, especially over the winter. (Some have argued that organized fermentation as both a food energy storage system and a reason to settle down was both an enabling and defining characteristic of what we think of as town-based civilization.) 1-2 hours per day, 6 days a week would not have been considered unusual at all in many settings, and could easily be closer to double that in some; given the way DF abstracts things out, that reasonably means that an ordinary dwarf will spend one to two days drinking a bit more often than once a month *if everything is fully efficient*; with long travel times, crowded stockpiles or dining areas, and particularly shortage of barrels dragging things out even longer. And this is a reasonable *historical* value given DF's weirdly pseudo-abstracted time system; fantasy dwarves are often depicted as even *more* booze-oriented than realistic historical folk. Note for instance that IIRC the DF dwarven liver is twice the volume of a human; that pretty strongly implies that they might be drinking twice as much...