It’s usually a mix of software limits and image-quality limits, not an immediate risk of damaging the phone.

Many smartphones can do longer exposures through different camera apps or computational “long exposure” modes, so the built-in camera app often sets the limit in software.

That said, hardware does matter. During long exposures, the sensor and electronics generate more thermal noise. As the sensor warms up, noise increases and image quality drops. This is especially noticeable on smartphones because their sensors and pixels are very small, so the signal-to-noise ratio is already more limited than on larger cameras.

In theory, any sensor could overheat if pushed far enough, but for normal photographic exposure times that’s not usually the main concern. The practical issue is that beyond a certain point, the image may get too noisy to be useful.

So the short answer is: long-exposure limits are often imposed by software, but they’re also influenced by the sensor’s size, heat, and noise performance.