How to calculate the total work hours from this time format?

Please upload a workbook or a representative cut down copy, anonymised if necessary. It is always easier to advise if we can see your request in its context.

Show a before and after situation with manually calculated results, explaining which information is data and which is results, and if it's not blindingly obvious how you have arrived at your results some explanatory notes as well.

To upload a file click the Go Advanced button at the foot of your post, look underneath the post area for the Manage Attachments section and take it from there.

That said you are of course complicating this system by not holding times as a decimal number which can be arithmetically manipulated without having to use string slicing to get to first base.