Some employers choose to have timed interviews. If your interview is timed, you will be alerted of the time limit prior to starting the timer. When you start the timer, you won't be able to pause it, and the timer will continue to run even if you exit the interview.
If your interview isn't timed, then you have the option to start your interview and come back to it at another time.
Unless stated otherwise, most interviews need to be submitted within four days from when you receive the invitation.