The session timed-out notice might appear when the internet connection is slow, indicating that the questions take longer to load. In this scenario, you should retake them using a faster internet connection. With this, take the DET as many times as you wish, as long as you don't take it more than twice in 30 days. In the meanwhile, you may take the TOEFL as many times as you like as long as you don't take it more than once every three days.