A soufflé is simply a hot air balloon. It stays up as long as there is hot air trapped inside it. Your 10-15 minutes sound reasonable, although it is a gradual process and starts as soon as they are out of the oven.
There are tons of kitchen lore about non-falling soufflés. These are either wishful thinking, or marketing trickery. The laws of physics cause a soufflé to fall quickly.
Your experience of 15 minutes is longer than my experience with chocolate soufflés, good job! Keep in mind that the hot air bubbles that inflate the soufflé have different permeability and structural strength depending on the type of soufflé. Savory cheese soufflés tend to stay "inflated" longer because the béchamel and cheese mixture forms bubbles that are both less permeable (they let less air out) and are structurally stronger (the cooled cheese keeps more of its shape than the cooled sugar-egg-and-flour mixture of the sweet ones).