Since the 48 have been done in under 4 days in summer and under 7 in winter (under 5 and just over 8 myself, respectively), the least number of days to do the Grid should be around 66? 78? At least under 100 days given somewhat cooperative conditions.
After I completed the (still only) Jan-Dec Calendar Year Grid in 2016, my effort was somewhat erroneously nominated for the FKT of the Year award. I commented on social media that the nomination was rather silly, that I was not trying for an FKT per se, but attempting “merely” to hike all 576 in one fun-filled year, AND that if a person wanted the honor of the FKT, they should do the 48 fast in Aug and July, thus completing the Grid in 10.5 months. Which is exactly what he did.
A few weeks ago I commented that the next thing would be to do them speedily in each month, thus having the “FKT” by the least total number of days. Surely someone will do it!!