As a working ceiling, keep the part of the title that has to be seen inside roughly fifty to sixty characters, and put the words that matter at the front. But the straight answer is that there is no fixed character limit, because what gets a title cut off is pixel width, not a character count. Google fits the title into a slot of about six hundred pixels on desktop, and since letters are not equal widths, two titles with the exact same number of characters can truncate differently. One full of wide letters like M and W runs out of room sooner than one made of narrow letters like i and l.
This is why the familiar sixty-character number is a rule of thumb, not a line Google drew. Google does not publish a character limit, and it reads the entire title tag regardless of what shows in the result, using the full thing for ranking even when the displayed version is shortened. As Google’s Gary Illyes has described it, title length is an externally made-up metric: technically there is some upper bound, but it is not the small number people quote.
And the edge is genuinely fuzzy, so the ceiling is a moving display behavior rather than a fixed rule. The cut differs between desktop and mobile, shifts with the device and even the query, and Google has been seen displaying much longer titles in some cases. Mobile is where the title gets clipped earliest, so that is the screen to size for, since a title that survives there survives most places. It is worth leaving a little buffer under the six-hundred-pixel mark too, because Google may append your site name and eat into the space.
Truncation and a rewrite are two different things. Google rewrites a title for its own reasons, mainly when the title does not match the page, is stuffed with keywords, or is generic boilerplate, and that can happen at any length. Length governs whether your title gets clipped. Relevance governs whether Google replaces it.