The Meta title is best when its about 65 characters (meta title tag recommended characters length) including white spaces and should be keyword rich.
However, keep in mind, the meta title is also seen as the link on Google search results. So it needs to be attractive and contain enough information to entice a Google search engine user to click on the link.
As for what would constitute spamming, probably using the main keywords more than once or twice could be seen as stuffing considering the limited space.
What's important is that you include your most important keyword/phrase as close to the start of your keyword title as possible.
The meta website title should be no more than 60 characters, including spaces, while the meta description should contain between 120 and 160 characters. I suggest putting the most important keywords in the front and the others in the back, as some search engines do not show the entire title or description.
I don't think it's character based at all, at least not to become stuffing anyway. I think they spot keyword stuffing when a word is repeated multiple times or is composed purely of keywords, even if they aren't 100% relevant.
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As you can see that's only 40 characters long but is clearly stuffing, it's nonsensical and screams spam.
I would stick between 50 to 60 characters at most. Not only for SEO factors but also just for readability. People have short attention spans and short headlines will attract more visitors if done correctly.