Although a bunch of studies found 25 hours or even much longer (median 25 hours), with a lot of variability from person to person, another later study suggested that early ones found so much variability due to residual light contamination and that the length was closer to 24.2 hours

Ensuring complete darkness for the duration of the study, they also found less variability from person to person.

There are also studies showing how much light is needed to change your circadian rhythm to a shorter or longer or different (think jet lag) cycle.