1991:
Video footage and dialogue reveal that the terrible accident that befell Victor Fries took place the year before the the main events of "Heart of Ice" which is set in August 1992.
This date also falls in line with Mr. Freeze's final appearance (in the Batman Beyond series), which takes place in 2040, 49 years after this incident:
Batman Beyond episode "Meltdown": "I've been like this for nearly 50 years, and I haven't aged a single day." - Victor Fries to Dr. Lake.