The movie is one of the most chilling horror movies ever made.
Deleted material (and Ramis' comment on Buddhist philosophy about ascending levels) showed that he was repeating that day for 10,000 years. Even without that idea, based on the skills he develops (learning French, playing piano, ice sculpture, etc.), it's been estimated at more than 33 years (which agrees with Ramis' non-philosophical estimate of 30-40 years).
What a horrific tale!