If 100 units corresponds to about three months... then that's about 32 months, or just over two and a half years, which is a bit less than Sulu stated, unless he was rounding up.
In the corresponding TNG thread ("Okuda timeline Canon?") I posted this:
"I have little doubt that Khan, who still considers himself to be the "king" of Earth, counted his days in exile (child's play with the help of his superior intellect) in solar days and solar years.
Also, Chekov didn't protest his "15 years" statement. Just this second an image popped up in my mind with Mr. Okuda taking Chekov's place and saying "Incorrect. It was only 12 years".
Khan: "Make that two [Ceti Eeels] for this character!"
And, for what it's worth, assuming they had intended 1,000 digits to equal one solar year, the difference between Stardate 3.143.3 ("Space Seed") and 8130.4 (ST II) is 14.99 years
The 1,000 digits theory also holds up pretty well for the time that elapsed between ST III and ST IV. After that things apparently moved "south" unless we could assume 11 years between ST II and ST VI (Stardate 9521.6)