I had a similar problem where I needed to detect if grandpa was out of bed too long at night. My Withings Sleep pad has IFTTT triggers for in bed & out of bed and I could set them for the hours I wanted, but no duration timer.
Here is how I solved it and it may be of use to you with your bathroom fan timer.
IFTTT trigger out of bed runs a Out of Bed Flow
This sets the "proceed" variable true and will set the "timeout" variable true after a timeout
If grandpa, gets back into bed before the timeout, the IFTTT trigger in bed fires and runs the In Bed Flow
This sets the "proceed: variable to false
There is a "connected" flow called "awake" and checks the two variables with an "AND" and sends the notification
In my limited testing, it does work