Realistically there has to be _some_ time limit. No one is going to sit in a room for 10 hours while you finish your test.
Why?
How strong is the argument that a student completing a test in 1 hour with the same score as a student who took 10 hours that the first student performed "better" or had a greater understanding of the material?
Sure. I doubt that if some test at the moment takes an hour then you're getting much extra benefit at the five hour mark. The whole point of the time compression is to spread the grades out - along an axis different to "competence".