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 exceed 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 look like on Mac.
The character limits are however lower for a rich push that contains an image.
Hence, copywriting plays a vital role, along with the timing and relevance of the content.