I regularly use boiling water as a timer. I set my whistling tea kettle to boil first thing in the morning, then go to the bathroom to brush my teeth and wash my face. I know that if I put 3 cups in the pot, I have approximately six minutes to complete my washing up; if I use 4 cups, I have 8 minutes. So, about 2 minutes per cup of water. (I always measure the water by cup when adding to the tea kettle, because I like my tea to have a consistent strength.)
The general rule seems to hold with an open sauce pan as well. I often have to boil one cup of water for a recipe; it takes two minutes.
It doesn't seem to matter whether it is winter or summer. My house is very close to sea level.
The reason that I know how long it takes is that I have a problem with walking away from an open pot of water and forgetting about it since it doesn't have a whistler. So I began setting the stove timer when I put on a larger pot of water for pasta.
The reason I found my way here, is that my boyfriend has started making tofu from scratch, which involves bringing raw soy milk to a boil and then holding it at boiling for 30 minutes. Soy milk burns and sticks easily. So I recommended a double boiler. He makes very large batches. So he is trying to boil 2 gallons of soy milk within an outer pot with a 4-gallon capacity. He stood around for a very long time waiting for it to boil and then got impatient and stopped the experiment after 50 minutes.
But I estimated that it probably would take over 90 minutes for the soy milk to come to a boil, based on my 2 minutes-per-cup rule, because he knows that he put one gallon of water in the outer pot. My calculation is 16 cups per gallon = 48 cups x 2 minutes per cup = 96 minutes. This may be off by a lot, since I don't know how the double boiler setup affects the time it takes, nor do I have experience with these larger volumes. (I assume that the soy milk will eventually boil in a double boiler, but I have not been able to find any evidence of that on the web.)
While waiting for more answers and data, I sent him a thermometer. :)