Hoy,
For me it all has something to do with the mission itself.
If it's a resque mission,it can be very short,however if it towing vessels into harbours,it all can be taken some time.
And offcourse if you want to go from one port to another,passing the ocean area's,that takes extra time.
If you make the times to short,of what you want,you limited the options which you can do.
Yep,and for some of us is a long version of a mission maybe boring.
Just look what you like,on the shipsim-game.
Marc