One hard limit, and the cuts that matter more
YouTube lets a title run to 100 characters and a description to 5,000.
Past those, it will not save the video.
The cuts that matter come much earlier.
Search results show about the first 70 characters of a title.
The column of suggested videos, and most of the phone app, show about 45.
Under a title in search, YouTube shows roughly the first 157 characters of the description.
Why the numbers are "about"
YouTube cuts by width on the screen, not by counting letters.
A title full of W and M runs out sooner than one full of i and l, and capitals take more room than small letters.
So treat a title that sits right on a line as cut.
Put the point first
"My honest review of the world's cheapest oven" is 45 characters, and it survives everywhere.
"I tried the cheapest oven in the world, and here is my honest review after a month" loses "honest review" on a phone.
Same video, same idea - but only the first title tells a scrolling thumb what it is.