The length of notification title and message depends on a number of factors including screen size, width, and input language. It may vary from device to device.
The total permissible data limit is 4 KB. If the number of characters exceeds the limit suited for a device type, the title/message may appear trimmed (followed by ...) when delivered on that device.
Here's an example of how a trimmed title/message may appear on Mac.
The character limits are lower for a rich push that contains an image. Hence, copywriting plays a vital role, along with the timing and relevance of the content.