To clarify what where I got this number: So, Saiyans
live longer than Humans, but according to Toriyama before this, it's they don't live "
too much differently" in terms of lifespan length to Earthlings. This, combined with the
Age Limit of 80 on their
Prime given to us by the Super Manga. Extrapolating from this, I took the oldest person to ever live IRL (Jeanne Calmont of France at 122, rounded down to 120), and called that the average lifespan limit of the Saiyans (which seems fair, as our own life expectantancy in comparison to her is usually at 80). Namekians according to DBZK, which we use,
Special Namekians, (which include's Daimao) live to about 500. Taking those two sets, I divided the Prime Age by the "Age Limit" of a Saiyan (.66666 repeating), and applied the ratio/proportion to the Namekian Prime equivalent (which, when applied to the 500 year number, is 300), and then combined the values (as Saiyans start aging again after that period, and thus so would Cell.)
Which means Cell, even without the Infinite Lifespan of Majunior/Daimao should still live to at
least 800. Potentially much longer thanks to Frieza's Cells, (as he has not seemed to have aged a single day between Broly and Namek, which was over 26 years since the start of canon, and neither did Cold, who is implied to be sunstantially older than Frieza.)