I did a 30 second long shot without the lens and checked the sensor and saw some dust on my sensor (even though camera has sensor cleaning). I didn't see any defects on photos yet, but is it something to consider and take actions ASAP?
Dust on your sensor happens to many of us, its not the end of the world.
If you're not seeing it on photos, I definitely wouldn't take drastic action. Order something like this rocket blower, set your camera to mirror lockup, and gently try to blow the dust off. You can also look at gentle liquid cleaning solutions.
If you are unable to remove the dust yourself and it becomes apparent in your photos, consider having your camera professionally cleaned by a shop or the manufacturer.
You won't see any dust from a long exposure without a lens. The dust becomes visible when you take pictures with a very small aperture, like f22, and without the lens you get a very large aperture, like f0.5. The exposure time is not relevant, the dust doesn't get more visible from a longer exposure.
What you see might be dead pixels in the sensor. In that case there is nothing that you can do about them.