It is not an uncommon thing to see writers worrying about their novels being too short or too long, and asking “how long should it be?” Well, here’s the answer. And it’s a really easy.
As long as it needs to be.
You can ask Dr. Google and it’ll spit out article after article telling you what the average word count for various genres are, but really, your story will determine how long it needs to be. And that's what this writing thing is supposed to be about, right? The story comes well before an arbitrary word count goal.
If your novel exceeds the 120k mark, it could be that you do have some fat that needs to be trimmed. If it doesn't serve your story, cut it. Alternatively, a manuscript hovering at the 30k mark might be in some desperate need of depth, of layers that need to be added to characters or setting. If either of these scenarios apply, cut or fatten up as you see fit.
However, that's not always going to be the case. If your 120k beast was already cut down from 180 and the story really does need all those words, then keep it. If your 30k work is already bloated at that length, maybe you need to accept that you've really got a novella on your hands. Yeah, you might get some pushback from agents or publishers if you're going the traditional route, but if you're self-publishing, there's literally no reason to adhere to someone else's expectations for word count. Just write your story as it needs to be written.