What are the benefits and drawbacks of timed online exams?
Ultimately, there must be some time limit, since the results must be submitted to be graded before the end of the course. For many courses in the humanities, the 'exam' is instead a paper to be worked on over a period of time (perhaps a week) and turned in by a given time. I suspect this is not what you are looking for. 

Instead, you are asking about an on-line exam given to the full class (or group) of students, where they accomplish the exercise in one sitting of some length. So, what should that length be? Traditional in-class exams are limited by the length of the class period, since some students may have a class directly afterwards. The final exam schedules that I have experienced also allot a fixed time interval, usually several hours. Again, different students with different class schedules may have other exams to get to. At the institutions I attended there were also appeal processes if a student had too many exams scheduled in a given time. For instance, each day in finals week had 3 exam slots, and you did not have to take 3 in a given day - they would move one for you. But each slot was a fixed length to make sure there was time to decompress, eat, do a quick review, and then get to the next exam. It was strictly forbidden for an instructor to go beyond the fixed time.

So, while you may feel that you want to give your students lots of time (just why I don't know - fit the material to a fixed time that one can reasonably expect students to work straight through), they have other demands on their time. By being 'nice' to your students you are in essence being greedy by taking away their time to meet other classes' obligations. Discuss with your registrar. Even in these unusual times, don't be a burden on your students.