Louis d'Orléans duke of Nemours was a widower for 38 years.
Abrecht Duke of Wurttemberg (Duke Carl's grand-father) was a widower for 37 years after the death of his wife ADss Margaret in 1902
Ludwig August of Saxe-Coburg-Gotha survived his wife Pss Leopoldina of Brazil for 36 years.
Charles X king of France survived his wife Pss Maria Teresa of Savoy for 31 years.
Carlos III King of Spain nearly reached your limit as he was a widower for 28 years.
You can do some research here: http://www.angelfire.com/realm/gotha/gotha/gotha.htm
Longtime widows are well-known in European royal history -- especially queens (e.g. Emma of the Netherlands, Maria Cristina of Spain, Elizabeth of the UK).
I was just wondering about the opposite -- i.e. kings or princes who survived their wives by many years and never remarried. By "longtime", I mean at least 30 years, such as Archduke Albrecht of Austria, Duke of Teschen. His only wife, Princess Hildegard of Bavaria, died in 1864; but he himself lived on until 1895 without ever remarrying.
His brother-in-law, Prince Luitpold of Bavaria, was also widowed in the same year: he, too, never remarried after the death of his only wife, Archduchess Augusta Ferdinand of Austria-Tuscany, despite surviving to the age of 91 (he died in 1912).
In more recent times, King Olaf V of Norway never remarried after the death of his only wife, Crown Märtha (who died in 1954 before her husband succeeded to the throne). He survived her by nearly 37 years.
Can anyone name other notable examples?