Just to add a little more info to the convo
If you're using dextrose, there is no fructose, it's only D-Glucose. Sucrose (table sugar), on the other hand, is a molecule of 1 fructose and 1 glucose.
Boiling kills microorganisms in the water and allows the sugar to dissolve in the water more quickly. Also, the warmer the temperature of the water, the more sugar it can hold in solution. You can dissolve sugar in room temperature water, it will just take longer. Boiling can also get the chlorine taste out of your water (if you're using tap water).
If you're just concerned with dissolving the sugar, 5 minutes or less of boiling would definitely be sufficient. However, if you're concerned with killing microorganisms, 15 mins would be safer. There are many microorganisms that can survive a 5 minute boil. I'm not sure how likely it would be that one of these would be in your water, but if you're worrying about contamination in the first place (which we all do), better safe than sorry. It's amazing the number of microbes that are floating around in the air. I imagine this is why some sources recommend 15 mins.
Also, yeast don't metabolize fructose directly. They must convert fructose to glucose. This takes energy, so if you're going for maximum efficiency, you're better off using dextrose.