Is it possible to set time limits in your forms? Can I set a form to be open just in certain periods, form one date to another?

Kind of the application of an examen where the students are allowed to repondo from Friday at 2 until Sunday at 6, and once they enter de exam they have 40 minutes to respond. Also they can only do it once.

Yes its possible. Have a look at the instructions here:

Conditionally Enable or Disable a Form Based on Time 

Note: its also possible in your form settings to disable the form on a specific date at a specific time. But you would have to enable it again manually. Which may not be what you want.