How to make a time mission

For instance, they have five minutes minutes to finish the mission. How can i make a timer and after the five minutes have passed, send the user to another scene because they lost?

If its just a timer you need, yeah its quite simple.

When the scene starts I assume you have some functionality to handle the ‘logic’ of the Level.
Just save the current time to a variable using: Time.time
For reference: http://docs.unity3d.com/Documentation/ScriptReference/Time-time.html

Then on it’s update you could check if the current time(using Time.time again) minus the start time is >300(5*60sec) then do your logic of ‘failing the level’ there

Hope that helps!