120 minutes is actually a pretty generous time. In my first playthrough I went in blind and managed to complete the game in 1h20min.
Before you start, go to the game settings and activate the timer. This way there's a clock during your playthrough to help you keep track of time.
If you pause the game the timer stops, so feel free to do so. If you waste too much time figuring a puzzle out you can also quit to the main menu and reload your last save to recover your time. The timer DOES NOT stop during cutscenes, though.
It is also possible to know how much of the game is left by pausing the game and checking the percentage on the top.