That really depends on the mode and of course how you play it. Standard rules are much shorter than advanced rules.
Standard rules should be between 2 and 3 hours. I'd prefer people to finish the main campaign on Standard rules in one session. This assumes that you don't rush but also you don't examine everything (during focus tests, people were playing 30 minutes long section of the game and some have still not finish it after more than an hour), get as many upgrades as possible and you don't fail (unless you play with "respawn and continue", that puts you back into the game immediately).
With advanced rules it is longer.
The game is released on July 19th.