A Google search result
Headlines are 30 characters each and the description 90. Google joins headlines with a vertical bar and will drop one entirely if it does not fit the width.
A Meta feed post
Primary text truncates at about 125 characters on mobile with a “See more”. The headline under the image is cut off around 27, which is where most ads lose the point.
A LinkedIn sponsored post
Introductory text truncates around 150 characters on desktop. Headlines run to 70, which is more room than Meta gives you and worth using.
A TikTok in-feed ad
The caption sits over the video and the interface eats the right-hand side and the bottom. Anything past about 100 characters is behind a “more”, and the safe area is narrower than people expect.