I'm tending to make them open book and allow plenty of time, for example.
I want to do that, but it's a conflicting set of pressures. In my day, exams, particularly finals, were often three hours, as they still are some other universities. We use 90 minute and 120 minute exams for 10 and 20 credit courses, as a baseline, with longer in special cases, which apparently (I wasn't in academia at the time) was a response to complaints about volume of exams. We then respond to dyslexia and other similar problems with an extra 25% or 50% of time. People with those allowances usually take their exam in an alternative, smaller venue which has invigilators laid on for longer.
But if you started having three hour exams, we'd need 4h30 for some people, which takes us into the problems of breaks: we've just vetoed a three hour computer-based exam on that basis. The departments which do "so much time everyone can finish" (a) have to explain why the extra-time students don't get extra time, which may not be easy and (b) resource the invigilation and venues, which is problematic.
I like the look of 24 hour take-home exams, in the hope that isn't long enough for for-hire writers to respond, but one can imagine a premium service arising, on standby for the day...