Good Question. I would think that engine cool-down would be the most critical period. Getting rid of the hot moist air in the crankcase as soon as possible after shutdown and replacing it with cool extra dry air would seem to provide the max benefit. I think that the benefit of operating the unit 24/7 would depend a lot on your local humidity.
In my case, I decided that spending $2000 to get rid of the rust in my almost new engine was not something that I wanted to repeat. Building this unit was a lot of fun. I'm just sorry that I didn't get the chance to use it.
To answer your 2nd question, I have not used the unit except to proof test it. You may want to contact the designer of the unit ,
davidbarker@msm.com , to see what kind of feedback he has received from those who have built his design and have used it for some time.
Best Regards, Bill Hogarty