For normal daylight scenes, a long exposure by itself is not likely to damage the sensor. The practical problems are usually a completely overexposed image, sensor/camera overheating warnings, and battery drain.

The real risk comes from extremely intense light concentrated on the sensor—such as the sun, lasers, or similar bright sources—not simply from the shutter being open for a long time.

In bright summer daylight, very long exposures will usually just blow out the image unless you reduce the light entering the lens. The usual solution is a neutral density (ND) filter; stronger ND filters allow much longer shutter times in daylight.

So: for ordinary scenes, long Bulb exposures are generally safe, but they won’t be useful in daylight without heavy light reduction. Avoid pointing the camera at the sun or other concentrated bright sources, and watch for overheating on very long exposures.