In terms of complexity, it doesn't add much. It all depends on your GOO (Great Old One), in fact, to determine what rules you're playing with in the end. And the additional rules don't make the game harder or more complex. They actually give the players other avenues to solving mysteries. Plus, setting up or tearing down the expansions doesn't add that much more time, maybe 5 to 10 minutes.
In terms of time. If you ignore the set-up and tear-down of the game and only focus on how long it takes to play, you don't need to worry about it taking significantly more or less time. The amount of time to play the game is baked into a few things: your Mythos deck and the GOO you pick. Each GOO has a doom track which serves as the amount of time you get to play before they wake up and cause the fourth mystery and access their win condition (or just outright win, in the case of Azathoth). The Mythos deck controls this by opening gates and moving the Omen. Players can construct a Mythos deck if they want to make it easier or harder, which allows you to alter and have control of the flow of the game. But you still need enough cards to satisfy what the GOO's card tells you for the deck. And, effectively, your Mythos deck is a hard time limit. When you're forced to draw from the Mythos deck and you can't, you lose the game.