Shadowgrounds takes around 10-20 hours to complete, depending on the play style. Below are the developers' approximations for each mission if using a fairly "normal" play style (i.e. checking some corners but not absolutely everything). This might be useful to know if you know that you have limited time in front of the computer. Use this as a general guide, i.e. "Mission 4 takes double the time as Mission 2".

Mission 1 - 15 minutes
Mission 2 - 30 minutes
Mission 3 - 30 minutes
Mission 4 - 60-75 minutes
Mission 5 - 45-60 minutes
Mission 6 - 45-60 minutes
Mission 7 - 60-75 minutes
Mission 8 - 45-60 minutes
Mission 9 - 60-75 minutes
Mission 10 - 45-60 minutes
Mission 11 - 45 minutes (incl. end cutscene + credits)

source:
http://shadowgroundsgame.com/new/index.php?page=troubleshooting#missionlength

I just played shadowgrounds for the first time yesterday, on medium. I mostly checked every corner, except on the large outdoor maps. Took me about 8 hours with a few bathroom & snack breaks.