120 minutes sounds about right for 4+ players. I suspect that the fastest possible play would be two to three players with good cooperative housekeeping, which might be under an hour.
I've played once with five players (three new), and it took about two hours from box open to completion (Hastur, by elder signs). There was definitely a "too many cooks" effect, especially with new players - we were hardly playing the game ourselves on many turns. Two hours does seem excessive, but that can be how it shakes out.
I have also played twice solo. The first took 90 minutes (Yig) and the second under 60 minutes, including setup from closed box. (I lose about 30 seconds trying to open the box. I might have to start greasing the stupid thing.)
Completion time is affected by other things too:
- Universal things - Players wandering off, explaining rules to newbies, over-strategizing
- Protectiveness - Trying hard not to get doom tokens or lose characters may cost you more real-world time than it is worth
- Big bad drawn - Ask an experienced player for details. This largely depends on elder signs.