How can I display the length of time my readers have spent on a particular page in Seconds to Human Readable Time

You want to measure the average duration of something, and you want to display it in human readable form (HH:MM:SS). The following query part can be used to transform a duration in seconds (herebey duration_seconds) to the proper format.

concat(lpad(string(integer(duration_seconds/3600)),2,"0"),
       ":",lpad(string(integer(duration_seconds%3600/60)),2,"0"),
       ":",lpad(string(integer(duration_seconds%60)),2,"0")) 
as avg_duration
Segentation
Behavioral