How to get the total length of Sequential Music

[Hans Sommer]

Hello,

I am relatively new to Lilypond. 
I try to check the total length of a SequentialMusic part, i.e. 


my_score_part = { c'4 d e f }

I want to get the result 4 quarters (or any other numerical representation)

and for 

my_score_part2 = { c'4 d e f g8}

should deliver 4 quarter and 1 eighth.

I found in the documentation the property:

 length-callback (procedure):

    ly:music-sequence::cumulative-length-callback 


but I don't know how to use it.

I would be very thankful, if anybody could provide an example.

Best regards


Hans