odoo-survey-addons/survey_input_duration/models/survey_user_input.py

from odoo import models, fields, api, _


class SurveyUserInput(models.Model):
    _inherit = "survey.user_input"

    duration = fields.Integer(
        string="Duration",
        compute="_get_duration",
        store=True,
        help="Duration in hours",
    )

    @api.depends("start_datetime", "end_datetime")
    def _get_duration(self):
        for input in self:
            start = input.start_datetime
            done = input.end_datetime
            if not (start and done):
                input.duration = 0
            else:
                input.duration = int((done - start).total_seconds()) / 3600