The Royal Book of Records: No Happily Ever After - less than a year of royal wedded bliss
Alexander III of Scotland and Yolande of Dreux (1285-86), married 4 months, 19 days
The widowed Alexander, who had buried the two sons, Alexander and David, from his first marriage to Margaret of England, wed again to get fresh heirs for the kingdom. Unfortunately, he went about the begetting with far too much enthusiasm. He was so eager to return to the lovely Yolande one fog-enshrouded night that he dismissed the weather concerns of his advisors and proceeded forth from Edinburgh to join her at Kinghorn, only to have his horse blunder off a cliff in the thick mist, and die in the fall. (They waited a bit to see if Yolande had conceived, eager to set up a nice regency, but alas, so she went home, remarried a Duke of Brittany, and had six kids.) This royal tragedy sent Scotland into a period of political turmoil. Alexander's sole heir was a four-year-old girl, Margaret, "the Maid of Norway", his only grandchild via his late daughter, also Margaret. Her claim was recognized and upheld (nothing better than a good regency), but sadly, the little Norweigian princess died en route to Scotland. Alexander had lots of cousins who then immediately fell to squabbling as to which of them should be the next king. (Those Scots do enjoy a good squabble.) Eventually this would lead to Edward I of England attempting to hammer the Scots, because they were dumb enough to ask him to stop the squabbling by determining who had the best claim, and then not liking his pick. Think Braveheart (only, like, actually historically accurate).
Edmund I of England and Aethelfleda of Damerham (946), married 5 months
Another second wife, another foolish king. Apparently not as enthralling a union as the above was, as Edmund, the grandson of Alfred the Great, was supposedly killed one night in a tavern brawl when he should have been about the business of begetting. Who knew kings went bar-hopping? Another version has an exiled thief murdering the king whilst he attended St Augustine's Day Mass in Pucklechurch (don't you just love some of the English place names?), Gloucestershire, as shown in the Georgian illustration. Yet another version has the thief, Leofa, party-crashing a feast and getting jumped by Edmund, whose bad temper then cost him his life. No email, you know, so sometimes the chroniclers just didn't agree on what happened. Though Edmund's first wife, Aelgifu of Shaftesbury, had done her heir duty before her demise, and he had two sons, Eadwig and Edgar, his brother Eadred (yeah, I don't know what was up with the *E* thing, either) succeeded to the throne (he's the dude who tangled with Eric Bloodaxe).
Henry VIII and Anne of Cleves (1540), married 6 months, 4 days
This arranged marriage turned out to be an inconvenient arrangement for both parties involved, as they took an intense dislike to each other on sight and never consummated their union. Henry went storming around muttering, "I like her not!". Fortunately, Anne was a good deal less stubborn and a good deal more amenable than Henry’s first wife, and she cheerfully agreed to a annulment based upon a past betrothal of hers to the Duke of Lorraine that Henry insisted made her ineligible to wed anyone. Henry was so grateful for the favor of a quickie divorce (he wanted a hot young trophy wife to show off to the court) that he termed Anne his “good sister", heaped lots of palaces and goodies upon her, and they then became the best of friends, with Anne happily received at court by her successor queen. The arranger of this marriage, kingly minion Thomas Cromwell, received a date with the axe, which took place on Henry's wedding day to his new inamorata, Catherine Howard.
James V of Scotland and Madeleine of Valois (1537), married 6 months, 7 days
The Auld Alliance between Scotland and France fetched up this fetching teen princess as a bride for the Scots king. It was said 19-year-old James fell immediately in love with her. However, Madeleine’s delicate Frenchie constitution could not cope with the aforementioned damp Scottish climate, and she succumbed to consumption at the age of sixteen. James being the sole surviving child from the union of Margaret Tudor and James IV, it was imperative he get over his grief and take a new bride posthaste to get him some heirs, so the moody young king was persuaded to wed the widowed Marie of Guise, whose son-bearing track record had already been proven.
Jane Grey and Guildford Dudley (1553-54), married 8 months, 23 days
These two teens were thrown together by ambitious Protestant families in a bid to seize the throne following the death of Edward VI so that his Catholic sister and heir, Mary, couldn't impose the old religion back on England. The nine days' queen was deposed and imprisoned in the Tower of London by Mary I, who seized the throne right back with a groundswell of popular support, and Jane and her spouse were executed a few months later when their families just couldn’t behave and kept trying to reinstate Queen Jane and King Guildford. Because they were clapped in the Tower of London less than two months after the wedding, this royal couple didn't live together for most of their brief union. Jane even refused Guildford's request to see her before his execution (which took place a day before hers).
Harald I of Man and Christina of Norway (1248), married 11 months
This royal pair never sat together on the Isle of Man’s thrones. After wedding and bedding in Christina's homeland, the Chronicle of Man states they set out for Harald’s little kingdom, only to be shipwrecked and drowned in a storm near a perilous tidal race (those cause big waves, eddys, and whirpools to suddenly develop) off Shetland. Worst honeymoon trip EVAH.
source: British Kings & Queens: The Complete Biographical Encyclopedia of the Kings & Queens of Great Britain - author: Mike Ashley Publisher: Barnes & Noble Books (2000) ISBN-10: 0760720347 ISBN-13: 978-0760720349
[paraphrased by my good self to be far more entertaining than the original, natch]