An example of an ESPHome based light (martin jerry MJ-S01) with an on-device timer

timed_light.yaml

substitutions:
  device_name: garage-light
  friendly_name: Garage Light
  default_delay: '900' # default delay of 15 minutes (900 seconds)
  pre_off_warning: '30' # pre-off warning period in seconds
  location: Garage
  ha_timer_number: input_number.garage_light_timer
  switch_name: Garage Interior Light Switch
  motion_sensor_name: binary_sensor.motion_garage_motion_2

esphome:
  name: ${device_name}
  comment: ${friendly_name}
  platform: ESP8266
  board: esp01_1m
  esp8266_restore_from_flash: true

globals:
  - id: delay_length_global
    type: int
    restore_value: yes
    initial_value: ${default_delay}
  - id: flash_count
    type: int
    restore_value: no
    initial_value: '0'

wifi:
  ssid: !secret wifissid
  password: !secret wifipass
  fast_connect: off
  domain: !secret domain
  power_save_mode: none
  ap:
    ssid: "${device_name} Fallback Hotspot"
    password: !secret wifipass

logger:
  baud_rate: 0

api:
  encryption:
    key: !secret esphome_key
  reboot_timeout: 0s
  services:
    - service: start_${device_name}_timer
      then:
        - script.execute: start_timer
    - service: update_delay_length
      variables:
        delay_length: int
      then:
        - lambda: |-
            id(delay_length_global) = delay_length;
            ESP_LOGD("main", "Delay length updated to %d seconds", delay_length);
            id(start_timer).execute();

ota:

# Device Specific Config
output:
  - platform: gpio
    pin: GPIO12
    id: relay1
  - platform: gpio
    pin: GPIO4
    id: statuslight

light:
  - platform: binary
    name: ${friendly_name}
    output: relay1
    id: lightid
    on_turn_on:
      then:
        - script.execute: start_timer
        - script.execute: flash_status_light
    on_turn_off:
      then:
        - output.turn_off: statuslight

binary_sensor:
  - platform: gpio
    pin:
      number: GPIO13
      mode: INPUT_PULLUP
      inverted: True
    name: "${switch_name}"
    on_multi_click:
      # single click
      - timing:
          - ON for at most 1s
          - OFF for at least 0.5s
        then:
          - light.toggle: lightid
          - script.execute: flash_status_light
          - if:
              condition:
                light.is_on: lightid
              then:
                - script.execute: start_timer
      # double click
      - timing:
          - ON for at most 1s
          - OFF for at most 1s
          - ON for at most 1s
          - OFF for at least 0.2s
        then:
          - switch.toggle: timer_disable
      # long click
      - timing:
          - ON for at least 1.5s
        then:
          - switch.turn_off: timer_disable
          
    internal: true
    id: switchid
  - platform: status
    name: "${friendly_name} Status"
  - platform: homeassistant
    name: "${motion_sensor_name}"
    entity_id: ${motion_sensor_name}
    id: motion_sensor

status_led:
  pin: GPIO5

text_sensor:
  - platform: version
    name: ${friendly_name} ESPHome Version

sensor:
  - platform: homeassistant
    entity_id: ${ha_timer_number}
    id: delay_length_sensor
    internal: true
    on_value:
      then:
        - lambda: |-
            id(delay_length_global) = int(id(delay_length_sensor).state);
            ESP_LOGD("main", "Delay length updated to %d seconds from Home Assistant", int(id(delay_length_sensor).state));

interval:
  - interval: 2min
    then:
      - if:
          condition:
            wifi.connected:
          then:
            - homeassistant.service:
                service: homeassistant.update_entity
                data:
                  entity_id: ${ha_timer_number}

script:
  - id: flash_status_light
    then:
      - output.turn_off: statuslight
      - delay: 500ms
      - output.turn_on: statuslight
  - id: start_timer
    mode: restart  # Change mode to restart
    then:
      - if:
          condition:
            - switch.is_off: timer_disable
          then:
            - delay: !lambda "return (id(delay_length_global) - ${pre_off_warning}) * 1000;"
            - logger.log:
                format: "Script has hit pre_off_warning: ${pre_off_warning} seconds"
            - repeat:
                count: ${pre_off_warning}
                then:
                  - output.turn_off: statuslight
                  - delay: 1s
                  - output.turn_on: statuslight
                  - delay: 1s
            - if:
                condition:
                  binary_sensor.is_on: motion_sensor
                then:
                  - script.execute: start_timer
            - light.turn_off: lightid
            - output.turn_off: statuslight

switch:
  - platform: template
    name: "Timer Disable"
    id: timer_disable
    optimistic: true
    on_turn_on:
      then:
        # this is kind of lame but i wanted a different 
        # flash and this was the easiest way for me
        - script.execute: flash_status_light
        - delay: 2.1s
        - script.execute: flash_status_light
    on_turn_off:
      then:
        - if:
            condition:
              light.is_on: lightid
            then:
              - script.execute: start_timer

time:
  # when the clock strikes 12, re-enable the timer
  - platform: homeassistant
    id: homeassistant_time
    on_time:
      - seconds: 0
        minutes: 0
        hours: 0
        then:
          - switch.turn_off: timer_disable

Updated to add a motion sensor in Home Assistant referenced by the substitution motion_sensor_name.

This will check to see if a motion sensor is showing occupied or (on) right before timer turns off the switch, if it shows occupied it starts the script again for the defined time.

If you're not using a motion sensor or want this functionality, then you'll need to remove any references to that in the binary_sensor section:

  - platform: homeassistant
    name: "${motion_sensor_name}"
    entity_id: ${motion_sensor_name}
    id: motion_sensor

and the conditional in the script flash_status_light:

            - if:
                condition:
                  binary_sensor.is_on: motion_sensor
                then:
                  - script.execute: start_timer