Just a simple question. How many missions are in the game? I just started playing it and all I know the game takes between 10 to 15 hours to complete.
A complete playthrough including optional side missions is 30 missions. I recommend not skipping the side missions, just makes life more comfortable as you can find some nice weapons in those missions. When you see the side missions on the map and you can choose between one of two optional side missions, don't worry about which one to take. Just pick one of them, the other one will disappear so you can't do more than 30 missions total in the end.
As for the time to completion, 10-15 hours is very rushed. On a first playthrough it's going to be more like 15-20 hours. I hope this helps, enjoy!