Calculate the Average Time Spent Viewing a Page

Hello all,  I'm trying to calculate the average time spent viewing a web page.  I have a column with the average time spent viewing a web page in a text format as '00:00:00'.  Id like to claculate the average time spent on each web page.  There are multiple entires of like web pages with different time spent durations.  My data columns are below:  Any help you can provide is greatly appreciated.  Many thanks!