When expires_in gets complicated you can simply use
:expires_in => lambda{ Time.now.to_i % 100 }

Code

class ActionController::Base
  def write_fragment_with_lambda_expire(key, content, options = nil)
    if options.is_a? Hash and options[:expires_in].is_a? Proc
      options = options.merge(:expires_in  => options[:expires_in].call)
    end
    write_fragment_without_lambda_expire(key, content, options)
  end

  alias_method_chain :write_fragment, :lambda_expire
end