Of the three bettas I'm currently watching, they each have different style and tendencies with their nest building.
Rescue-betta (red/blue VT) will build them in the far corner of his tank if you leave him alone for a couple days, but they are usually small and he doesn't seem to put much effort into them. However, if you place a section of dried banana leaf in the tank (which floats) he will have built a bubble nest under the entire length of the leaf in several hours, which then spreads out around the leaf.
Black HM - is pretty scattered in his bubble nesting tendencies. He has a strange habit of blowing several very large bubbles (they're almost the size of my pinky nail) and sticking them together, and then going and repeating the process again in a different part of the tank. They don't really last long, and he looses interest in them quickly. He also doesn't care about the banana leaf sections when I put them in his tank, they're fun to peck at and hide under, but he doesn't build bubble nests under them. He's much more active than my other bettas, and seems much more interested in dancing around and flaring.
Xiphoid (Red VT with elongated pectoral fins) This is actually my mate's betta, I'm just keeping him right now in a quarantine tank before he goes off to his permanent home in a 10 gallon of his own. He is the most prolific bubble-maker of the three, although it took him a couple weeks before he fianlly settled down and started blowing bubbles. His nests are positively huge! He builds them up over the course of a couple days, and they cover better than three-quarters of the surface of the tank he's in. I sometimes wonder if he's going to have any space left to breath at once he gets going with his bubbles. It's kind of sad when I do water changes, because I end up destroying his nest, but he's acclimated enough now that in a couple more days, his monster nest will be back.