Set a default expires_in value for memcache

I bet there is an easier way to do this, but here's what I came up with. I didn't want to cache items forever (which is the default with memcache and rails). I wanted to cache items for 6 hours, max. To do this, I wrote a patch and if the :expires_in option is missing, it will set it to 21600. I would love to hear if there is a better way...

module ActiveSupport
  module Cache
    class Store
      
      private
      
      # Original code from ActiveSupport::Cache::Store
      # 
      # def expires_in(options)
      #   expires_in = options && options[:expires_in]
      #   raise ":expires_in must be a number" if expires_in && !expires_in.is_a?(Numeric)
      #   expires_in || 0
      # end
      
      def expires_in(options)
        expires_in = options && options[:expires_in]
        raise ":expires_in must be a number" if expires_in && !expires_in.is_a?(Numeric)
        expires_in || 6.hours
      end
      
    end
  end
end