I am a big fan of playing and experimenting with time. Arround 2 years ago i bought a HAIDA 10x ND filter, since then I’m enjoying shooting moving objects like clouds, people, cars etc next to architecture. The maximum time I have tried so far was arround 30 minutes using 2 of the x10 ND filters attached to eachother. Pixel errors occured on images taken longer than 20 minutes with my Canon 5D III. But for the most tasks shorter durations let you get the better results anyways. My favourite thing to shoot with ND filters are definitly moving skies, especially when reflected on water. There is a special sky needed to get a good contrast between cloud and sky. Shiny white single clouds that move towards the camera in front of a dark blue sky, this is a good setup to spend some longer time to capture a lot of time in a single image.
Another useful purpose for a ND filter is when you want to get rid of people crossing your picture. Taking a picture of 5 minutes e.g. will only show people who are sitting or standing around. Moving people will just disappear magically.
Another hint for the post work of moving cloud images is to turn down the luminance of the blue color (blue sky). This will higher the contrast a lot and works even better with black and white.
If you would like to try long time exposures at daytime, here is what you need besides your camera: ND filter (neutral density), a tripod, a remote control to set up the time and of course a lot of pacience.