How long can I expect a playthrough of Black Mesa to take, on average, compared to Half-Life?
It is almost entirely up to you and how you play the game. If you take your time and immerse yourself, it will take longer than if you try and speed-run it. Either way, you can expect it to take less than an equivalent playthrough of Half-Life. Black Mesa has in general more cut and reworked sections than new areas added.
