The default Skydome would not be able to do this without quite a bit of modification, however there is a community content Skydome available on the forums that will let you adjust the sun speed using a float curve. I have another project available here that takes the skydome a bit further (it also has an ocean surface), allowing more customization and much darker nights, feel free to download it and migrate the content over to your project.
If you want to keep the default skydome, the night sky does not update by default when you rotate the sun using the default skydome, pressing the “Update Material” option will cause the night to become quite a bit darker.
To lengthen it, I would just slow down the day night cycle clock mathematically. To write a day/night cycle clock, I’d recommend TheFaceGrabber’s YouTube videos: