When Must One Close His Gemara?

The Chofetz Chaim, zt”l, was unusually scrupulous that his weights and measures should always be exactly correct. Despite his zealous care not to waste a moment unnecessarily, adjusting his scales was the one business need that took him out of the beis midrash every weekday without fail.
Each day, he would close his gemara to visit his store and check that the weights and measures were exactly correct. He would never rely on his having checked them the day before, since he saw it as his holy duty to be absolutely certain that he was not cheating anyone, even for one day.
Reb Yosef, zt”l, a student of the Chofetz Chaim and also the ironmonger of Radin, recounted an amazing story which demonstrates the care that his rebbe took in this area. “The Chofetz Chaim gave me the honor of making his weights for him and replacing them when they wore out, but he would not allow me to make the marks signifying the exact position of each weight. This task he left to himself. If I had not seen how he dealt with those weights I would never have believed it.
“It took him hours to make one siman on a weight. In order to ensure that the weight was exactly correct he would spend hours before he was finally satisfied that it could be used.”
“It is well known that I was a very poor man in those years,” Reb Yosef continued. “But I tell you now that I would not have agreed to mark those weights with the scrupulous care of the Chofetz Chaim if he had paid me twenty five ruble an hour! Even for what was a veritable fortune for me, I would never have been able to replicate the intense focus that the Chofetz Chaim put into what otherwise would have been a simple task, with a much less honest result.”