Here is my Automation:

alias: Update Last Shower Time
description: ""
triggers:
  - entity_id: binary_sensor.showering
    from: "on"
    to: "off"
    trigger: state
actions:
  - data:
      datetime: "{{ now() }}"
    action: input_datetime.set_datetime
    target:
      entity_id: input_datetime.last_shower

You need to create a ‘DateTime’ helper called “Last Shower” This is where the last shower time will be stored (and can be changed)

You need a ‘Statistics’ Helper too. Call it “Average Bathroom Humidity” and base it on whatever humidity sensor you have. Make it “average linear” then set Max Age to 1 hour. (leave other options default)

Make these template sensors too:

Template Binary Sensor called “Showering”:

{{ (states('sensor.bathroom_sensor_humidity')|float - states('sensor.average_bathroom_humidity')|float) > 9 }}

Template Sensor called “Time Since Last Shower”:

% set diff = now() - as_datetime(states('input_datetime.last_shower')).astimezone() %}
{% set days = diff.days %}
{% set hours = (diff.seconds // 3600) %}
{% if days > 0 %}
  {{ days }} day{{ 's' if days > 1 }} {{ hours }} hour{{ 's' if hours != 1 }}
{% else %}
  {{ hours }} hour{{ 's' if hours != 1 }}
{% endif %}

Let me know if you hit any snags!