A meta description should ideally be between 120 and 160 characters to ensure it appears fully in search engine results without getting cut off. Most sources recommend keeping it around 150-155 characters as a good balance to convey the message concisely and attract clicks. It is also important to make it unique, relevant, and engaging to users, including keywords where appropriate. Lengths shorter than about 70 characters may be insufficient to convey useful information, and lengths beyond 160 characters often get truncated in the search results.

For mobile devices, shorter descriptions around 120 characters are preferable since the display space is smaller. Google generally measures meta descriptions not only by characters but also by pixel width, with about 920 pixels (roughly 155-158 characters) being the desktop maximum to avoid truncation.

In summary, aiming for a meta description length of 120 to 158 characters is recommended to optimize its visibility and effectiveness across different devices and search engines.