If the registered hours are linked to the System.User entity you can use the $CurrentUser keyword/variable to retrieve all the hours registered for the currently logged in user in one flow.
I doubt you can do it with what you have but if what you are trying to do is reuse some of your code, you might want to break your microflow that retrieves hours for each month for all users and do the following:
- microflow 1: gets a list of hours worked, per month, per user.
- microflow 2: uses microflow one and for each month, adds the hours worked by all users to get the result of your initial microflow.
- microflow 3: uses microflow one together with your microflow that retrieves the user currently logged in to get the number of hours for this user over one month.
Hopefully this has to do with your question :).