Search engines regularly replace it with a passage from the page that better matches the query, which is not a failure — it means the page answered something specific. Write descriptions for the queries where you want to control the pitch: commercial pages, comparison pages, anything where the click decision is close.
Around 150-160 characters is the practical limit on desktop, less on mobile. Say what the page uniquely offers rather than restating the title, and avoid auto-generating them from the first paragraph across a whole template, which produces descriptions that repeat the same opening sentence thousands of times.