I don't think Tableau will recognize your duration time format. You'll need to parse it, e.g.
INT(SPLIT([duration _string],":",1))*60 + INT(SPLIT([duration _string],":",2))
If your duration strings aren't always HH:MM:SS you'll need to guard against nulls using ZN or splitting from the end using negative token numbers.
Thanks a lot Zach Leber for your help. Its worked straight away.
Hope you will follow us and can help us with other questions as well. We're not professionals, but patients. Still we hope we can get our own Patient Driven Research up and going were nobody else is doing it for us.
Your input helped us one step into the right direction
Glad to help Rogier. I happen to work for the Broad Institute, which recently spun out our very successful patient-driven research initiative into its own non-profit called Count Me In. It's cancer-focused for now but may give you some ideas or contacts. I'll follow you so you can e-mail me if you have more questions.