Saw this mentioned a few times in another thread so I decided to see what happens in my kitchen. I use a refractometer and collect a few tablespoons in a bowl then use spoon to stir and dose. These are the results:
8:50 pulled 3 tablespoons from kettle
8:54 reading 12b at 73 degrees
9:08 12.2b at 67
9:40 12.4b at 67
9:56 12.4 b at 67
10:15 12.6b at 67
point 2b is about .85 gravity points so over the course of 1 hour I would have seen about a 1.7 increase in the gravity of the solution due to evaporation. About 1/3 of a tablespoon was used per reading. Since I was able to obtain a suitable temperature in less than 5 minutes, I will continue be ignorant of evaporation. While writing this I realize I should have put an open hydrometer size glass in the freezer to compare, next time.