Defining, for the sake of convenience, this to be any female who inherits a throne in her own right -- whether or not she actually reigns or rules after succeeding (she might be only a minor under a regency, deposed or forced to abdicate for various reasons) -- I was interested in those periods in European history, over the last 500 years, when there was at least one queen regnant somewhere in the continent.
I say this because there was one somewhere, during the entire 16th century. Spain and Portugal, for instance, have always permitted their thrones to be inherited by females. But until Isabel I of Castile none actually ruled (their husbands ruled in their stead, and in fact have been officially acknowledged as kings, not just kings consort).
Scotland has allowed this since the nominal accession of Margaret ("The Maid of Norway). And it goes without saying that if a female can inherit a throne in her own right, she can also transmit succession rights at least to her male descendants (e.g. Marjory Bruce). And in England, females have always had the right to transmit succession rights at least to male descendants (e.g. Adela of Normandy, Holy Roman Empress Matilda).
With all this in mind, I count Juana la Loca of Spain as continuing female inheritance in the 16th century, despite being declared insane and unfit to rule, and even imprisoned -- just like her descendants in Bavaria (the so-called "Mad" Kings Ludwig II and Otto). Given the deaths of her only brother and older sister without surviving issue, she rightfully inherited the thrones of both Castile and Aragon, before dying in 1555.
Meanwhile, Mary Stuart came to the Scottish throne in 1542 and her namesake, Juana's niece Mary Tudor came to the English throne in 1553 (the first uncontested queen regnant of England). The latter, of course, was succeeded by her half-sister Elizabeth I, who ruled until her death in 1603.
Thanks to the Glorious Revolution, female rule returned later in the 17th century; but I'm not sure about the record elsewhere in Europe.
The 18th century, however, has been called the "Century of Women" in Russia, thanks to the number of empresses who reigned. To be sure, neither of the two Catherines had a genealogical claim on the throne. But both ruled, and unlike other regents, have effectively been regarded in history as queens in their own right (as opposed to queens consort). The fact that each as a postnominal number indicates that historians regard them as empresses regnant. However, that century in Russia did not have an unbroken record of female rule, since there were several czars who reigned.
That being said, in Austria Holy Roman Empress Maria Theresia ruled, despite being technically only a consort (her husband was elected Holy Roman Emperor) So perhaps the period 1740-1780 could count as the one where there was at least one queen regnant somewhere in Europe.
Female reign returned to Spain in 1833, with the death of King Fernando VII and accession of Queen Isabel II -- notwithstanding the Carlist wars that erupted afterward. Given the accession the following year of Queen Maria II of Portugal, the reign of Queen Victoria of Great Britain from 1837 to 1901, and the fact that the Netherlands was headed by a queen regnant from 1890 to 2013, and that both Denmark and the UK are still headed by queens regnant, the period since 1833 has witnessed the reign of at least one queen. I believe it's the longest in European history.
Can anybody fill me in on other periods? What about before Isabel I of Castile?
There were two Queen Regnants in Sweden. Christina (1632-1654) and Ulrika Eleonora (1718-1720).