My experience has been that the approval just expires, so the flow (depending on what Run After actions you've set for the following action) will usually end there.
I'm thinking that you could have it trigger another flow that checks a stored record (with boolean yes/no and some kind of referenceID for that item) every day until that record is either changed to yes for that item or 30 days expires.
I've seen some others online deal with this in a number of different ways, including one that re-sends a new approval when that one expires but the setup for it is a bit more complex.
