_form.html.haml

= semantic_form_for @site do
  = form.inputs do
    = form.input :wait_duration

duration_conversion.rb

module DurationConversion
  extend ActiveSupport::Concern

  included do
    attribute_method_suffix "_duration"
    attribute_method_suffix "_duration="
  end

  # Handle *_duration for method_missing.
  def attribute_duration(attribute_name)
    ChronicDuration.output(send(attribute_name), :format => :long)
  end

  # Handle *_duration= for method_missing.
  def attribute_duration=(attribute_name, value)
    send(:"#{attribute_name}=", ChronicDuration.parse(value))
  end
end

Gemfile

gem 'chronic_duration'

site.rb

class Site < ActiveRecord::Base
  include DurationConversion

  # t.integer :wait
end