From my observation, the ending of a SoL seems to depend on whether the story itself, such as its tone. There are SoLs that are so random that it can virtually end in any note since life goes on forever. Sayonara Zetsubou Sensei for instance is a slice of life gag manga that's really bizarre and random that even if the writer stopped writing it midway it wouldn't really feel inconclusive. He only made a really weird ending to at least end in a bang.
On the other hand, there are slice of life stories that focus on certain themes such as growing up, life, and youth and the story ends with the characters changed in some way. Goodnight Punpun is a slice of life coming of age manga that focuses on how relationships can affect your decisions and how those decisions affect your outlook and how you face life. It ends with Punpun grown up and learning stuff.
I currently have a slice of life comic that I doubt would end because I used a more lighthearted, day-to-day tone. Overall, it's all about what kind of Slice of life you want to tell and what do you want your readers to take away from it.