Hi @Lars_Andersson,
The way I would solve this is to first store the time once the value is above the threshold you set; you can store this time in workflow storage or within an attribute, whichever you prefer. Then, once the time has dipped back below the threshold, do a comparison of these times and get the difference. You can then store this time value in a device attribute, individually as one single time, or you could do a cumulative count within the attribute.
Let me know if this meets your requirements!
Julia