On desktop, it should show how many songs are in your 'Liked Songs' playlist as well as the total combined length of all songs in the playlist, like other playlists do. While mobile does show the number of songs, it doesn't show the total length.
There is a workaround for this, which is to add all liked songs to a new playlist, however, this only works if you have a small amount of songs in your liked songs. Currently, if I use the ctrl+A command, it only selects a small portion of the songs, and if I select a song at one end of the playlist, then shift+click on the other end, there are many gaps in the selected songs. It seems to select a random amount, anywhere from 50% to 15%.