I´ve come across quite a few missions where according to mission description there is some time critical element. For example "Save ship X wich is under attack by Y"....you can wait and sit arround for ages is those missions...the ship you´re supposed to save never explodes and leads to a failed mission. Same with missions where some enemy has put explosives on the ground....those explosives NEVER do anything....you just leasurly disarm them.....they should explode after a certain time and lead to mission failure.
There are many more missions where a time critical element is impliied but not really executed that way.
On the other side...there are a handfull of missions where there really is a timer....(for example hold position until reinforcment arrives in X)...but those are to easy most of the time.