The Arkham Horror Statistics project has compiled data from over 15,000 player-submitted games, including the amount of time it took. As of this writing, the project has calculated that a game of Arkham Horror (not including setup) takes:
96 minutes, plus 18 minutes per player
More accurately, the game takes 96 min ± 8 min, plus 18 ± 2 min per player. Thus the game is expected to take the following amount of time:
# | Min | Max | Range
1 | 104 | 124 | 1h 44m - 2h 04m
2 | 120 | 144 | 2h 00m - 2h 24m
3 | 136 | 164 | 2h 16m - 2h 44m
4 | 152 | 184 | 2h 32m - 3h 04m
5 | 168 | 204 | 2h 48m - 3h 24m
6 | 184 | 224 | 3h 04m - 3h 44m
7 | 200 | 244 | 3h 20m - 4h 04m
8 | 216 | 264 | 3h 36m - 4h 24m
It's likely that these numbers are weighted towards those who are already familiar with how to play the game, since hardcore fans are more likely to find this statistics project. If you have players who don't know the rules well (or you don't have anybody at the table who knows the rules well, the game will certainly take longer.
Anecdotally, I'll say throw in 30 minutes if one or two players don't know the rules, 60 minutes if most players don't know the rules, and 90 minutes if nobody has ever read the rulebook before playing. Add 30 minutes if somebody at the table knows most rules, but requires frequent lookups in the rulebook.
And again, these numbers do not include setup. If you are punching out pieces of cardboard or shuffling every deck, the game will certainly appear to take much longer.