We don't have much concrete details, but it is less than 100 years because Teysa is said to be 112 in her newest story and she was alive and influential in the original book. Though we get no official age for her in the original timeline she can't be too young because she is rather accomplished.
We see some of the same characters between the two blocks (Isperia the Inscrutable/Isperia, Supreme Judge, Teysa, Orzhov Scion/Teysa, Envoy of Ghosts, Ghost Council of Orzhova/Obzedat, Ghost Council, Niv-Mizzet, the Firemind/Niv-Mizzet, Dracogenius, Rakdos the Defiler/Rakdos, Lord of Riots, and Borborygmos/Borborygmos Enraged), but they are all inherently long-lived characters, making it hard to judge the number of years by a typical Earth human's lifespan. We also have Jarad, Golgari Lich Lord as a story bridge between the Ravnica Block and the Return to Ravnica Block, but he is a zombie, so it is again difficult to judge the amount of time between these two blocks.
Any ideas, or is this just a mystery?