Is there a location in SharePoint 2013 where the users can get an overview of the Flexitasks that they have yet approved/rejected? Or is it somehow possible for users to see this?
If you are talking about a list that displays the tasks that have completed and not completed, that is the 'Workflow Tasks' list. It can be found in SharePoint when you click on 'Lists'. It is usually listed near the bottom since the lists are in alphabetical order. You can sort by user, complete/not started, etc.
if you mean one central place for whole sharepoint farm, then I'm not aware of such possibility.
you configure a task list per every workflow, that's the place/list where all the tasks are stored (not necessarily only flexi tasks).
you can share the task list for all the workflows within the site.
so you can direct users to go to the task list and make a filter for their own tasks.
Retrieving data ...