The first book in the A Song of Ice and Fire series, A Game of Thrones, was released in 1996, with the second book, A Clash of Kings, following shortly after in 1998.
After this, Martin completed the third book in very quick succession, with A Storm of Swords coming out in 2000.
However, A Storm of Swords marked the last quick release novel, as the fourth book, A Feast for Crows, came out in 2005.
This was followed by A Dance with Dragons, which came out in 2011, and sadly in 2020 no sixth book has arrived.
According to various sources, the sixth book, The Winds of Winter, will take place further north than any of the previous book, and the Others will make appearances.
According to Martin, the fifth book, A Dance with Dragons, did not have as much story as expected, meaning these storylines will likely be resolved at the beginning of the sixth book.
Martin also revealed in a an interview that a Victarion Greyjoy chapter will begin give minutes after the end of A Dance with Dragons, and take place on the eve of Ironborn’s arrival in Slaver’s Bay, while another sample chapter from Martin showed Arianne Martell heading to Griffin’s Roost with a young man called Aegon VI Targaryen.
“Things get worse before they get better, so things are getting worse for a lot of people.”
But with regards of A Dream of Spring, Martin has said he is only firm about ending the series with the seventh novel “until I decide not to be firm,” according to Entertainment Weekly.
He has said he knows what will happen broadly with his main characters, however according to a 2011 interview in Entertainment Weekly, he did not want the series to end in too different a way from his own ideas.
This could prove difficult, as the ending to the HBO series Game of Thrones, based on his novel series, was pretty divisive, with some fans declaring it the worst ending ever.
As a result, Martin will likely have to deviate from this ending in his final book, which he has confirmed he would not start until he had finished The Winds of Winter, which is still in its process.
Therefore, sadly the answer to the question of when A Dream of Spring is released, is currently non-existent.
Given how long many of the recent books have taken years to write, and the fact that there are more Game of Thrones stories to be made into TV series, the likelihood is A Dream of Spring will likely not arrive until at least the end of the decade.
However, with Martin being a talented writer with many ideas, there’s always an option this amount of time could be shortened.