There is no way to pass a parameter from Google Assistant to Stringify. All you can do is trigger a Stringify flow by name.
Neither Google Assistant nor Alexa can yet be told to take an action "in XX minutes". Yes, it's a surprising shortcoming, and a frequently requested feature.
You can of course include a fixed timer in a Routine or a Stringify flow, but that isn't what you want.
So unfortunately you're asking for something that sounds like it should be possible - but unfortunately it isn't at the moment. Maybe next year.
If you feel ambitious,you could do something complicated like this:
- Create multiple Stringify flows that each set a variable to the number of minutes of a particular timer you would commonly use, e.g., one for 7 minutes, one for 15 minutes, one for 20 minutes etc.
- Assign a name Google Assistant routine to call each one, like "Set Minute Counter 20".
- Set up your timed Stringify flow to reference the variable.
- So then you could say something like "Ok Google, Set Minute Counter 20", followed by "Ok Google, Delayed Outlet On"