This is my guess but I suppose it may happen in a particular scenario where you keep the shutter open in bright sun with no ND filter, and leave this open for 15 minutes. You could literally cook your sensor in such a circumstance, I'd imagine.
The right exposure time to be used is really 15 minutes, I don't think there'd be an issue. I know a lot of overseas photographers who shoot loads of star trails, and some of them keep the shutter open for way longer..
My opinion... it will not matter if the long exposure are at night and not pointing directly to the sun. However, it might not be too wise for too long an exposure, as heat will build up and noise will occur. Many people would rather take multiple shots at shorter exposure, then stack them up during PP.
If it needs 15min to 'collect' enough light then it's alright. Do your exposure calculation (e.g. test shots at high ISO, wide open aperture), examine the results then calculate the stops down.
As mentioned, it's rather a question of the sensor heating up.
you are going to use nd110 at nite?? usually after sundown the exposure times go from 15s & up already, without ND filters, depending on the scene. Why the need for so long exposures? I've tried shooting star trails for up to 30min before & I've seen people with 2hr long exposures as well. Don't think any damage was done to the sensor... battery life is the bigger concern here.