JRuby (tested on 1.7.4 and 1.7.9) exits after 10 seconds. MRI exits after 5 seconds.

jruby_timeout_process.rb

require 'timeout'

puts "Starting at #{Time.now}"
begin
  Timeout.timeout(5) do
    `sleep 10`
    puts 'after sleep'
  end
rescue Timeout::Error
  puts 'timed out'
end
puts "Ending at #{Time.now}"