The DecPlay style of song sheets (known as Pianotabz) are designed to enable rapid progress learning songs that you are familiar with. If you don't already know the song you want to play, you can play the demonstration lesson of the song within the course, until you are familiar with it. This enables you to play the song without needing to read a time signature or written rhythm of each note. You play the melody rhythm by following the lyrics. 

Relying on your familiarity with the song and following the lyrics instead of reading rhythm in the written music, is one of the reasons that the PianoTabz system of notation is so fast to learn compared to traditional notation. It is not designed for complex classical music or music that you are not familiar with.

DecPlay's PianoTabz song sheet format is a written form of how I play by ear and purposely does not involve reading rhythm. It is ideal if you wish to learn to play songs on piano quickly and easily from beginner to advanced level but is not designed to teach traditional notation.

The reason that the timing is not given in the music, is this would significantly slow down the learning process as you would have to read the timing for every single note. 
If you are familiar with the tune, most people find it easy to Play the timing of the melody by reference to the words of the song. I don’t recommend students try playing songs that they are not familiar with as this makes life really hard and misses out on the motivation which is gained by playing a song that you like already. 

The timing of the chords is usually a constant number of beats e.g. four beats and again, the timing of the chords are played by following the words of the song.

In addition, if you play along with a demonstration of the song, this also shows you the rhythm. You can slowdown the videos e.g. to half speed to make this easier to play along to, when you are learning.