Several years ago, I tinkered around with various solutions for making long-lasting soap bubbles and films. The best of these, heavy in glycerin, produced two foot diameter bubbles which lasted about twenty minutes before they succumbed to evaporation and drainage. Although glycerin dramatically retards evaporation, it is expensive and messy, and solutions made with it really ought to sit quietly for a day or better before they are used to allow for proper mixing. This situation doesn't lend itself to impromptu experimentation, so I was delighted when I ran across an apparatus designed by Dr. Maarten Rutgers which produces large soap films which last indefinitely, using ordinary dish soap and a few dollars worth of materials.
Smaller films can last for hours.
Dr. Rutgers has very detailed instructions available, along with several articles describing the physics of soap films, which he uses to visualize fluid flow. Briefly, the device consists of a large loop of fishing line. The bottom of the loop is attached to a small weight, while additional pieces of line are attached to the sides of the loop which allow it to be pulled open. Solution from the reservoir constantly flows down the loop, which produces a soap film when opened. The weight pulls the loop closed when the side pulls are released, allowing a new film to be formed quickly.