I just want a way of knowing when the mash really was done.

You'll have to define your "done" criteria. Like many reactions in nature, I suspect that you don't get to 100-percent conversion linearly, but approach it asymptoticly: 60 minutes to get to 90% conversion, another 60 minutes to get 99.9% conversion, another 60 minutes to get 99.99%,

*etc*. At some point you reach diminishing returns. I cut mine off at 60 minutes and accept whatever the small efficiency hit is.