I'm working with LS alpha 5 and 2.4 (last stable) both gave the same error, but alpha allowed me to connect more machines
I tried with 3600, I am still getting the same error, plus now I find i/o timeout on FB and LS print this error
{:timestamp=>"2016-09-20T15:33:34.077000+0200", :message=>"PeriodicPoller: exception", :poller=>#<LogStash::Instrument::PeriodicPoller::JVM:0x15e1af9f @task=#<Concurrent::TimerTask:0x45e557e2 @observers=#<Concurrent::Collection::CopyOnNotifyObserverSet:0x5f4f22c8 @observers={#<LogStash::Instrument::PeriodicPoller::JVM:0x15e1af9f ...>=>:update}>, @timeout_interval=60.0, @running=#<Concurrent::AtomicBoolean:0x14357ba2>, @StoppedEvent=#<Concurrent::Event:0x7b806481 @set=false, @iteration=0>, @execution_interval=1.0, @do_nothing_on_deref=true, @run_now=nil, @freeze_on_deref=nil, @executor=#<Concurrent::SafeTaskExecutor:0x5d7f405b @task=#<Proc:0x2359d488@/usr/share/logstash/logstash-core/lib/logstash/instrument/periodic_poller/base.rb:52>, @exception_class=StandardError>, @StopEvent=#<Concurrent::Event:0x1c0e432a @set=false, @iteration=0>, @value=true, @copy_on_deref=nil, @dup_on_deref=nil>, @peak_threads=139, @peak_open_fds=248, @metric=#<LogStash::Instrument::Metric:0x64528846 @collector=#<LogStash::Instrument::Collector:0x1ca06c48 @agent=nil, @metric_store=#<LogStash::Instrument::MetricStore:0x4388710f @store=#<Concurrent::Map:0x2e181062 @default_proc=nil>, @structured_lookup_mutex=#<Mutex:0x40611198>, @fast_lookup=#<Concurrent::Map:0x3001b16a @default_proc=nil>>, @observer_state=true, @snapshot_task=#<Concurrent::TimerTask:0x3001be6c @observers=#<Concurrent::Collection::CopyOnNotifyObserverSet:0x5e74230e @observers={#<LogStash::Instrument::Collector:0x1ca06c48 ...>=>:update}>, @timeout_interval=600.0, @running=#<Concurrent::AtomicBoolean:0x2b667dd8>, @StoppedEvent=#<Concurrent::Event:0x16a94406 @set=false, @iteration=0>, @execution_interval=1.0, @do_nothing_on_deref=true, @run_now=nil, @freeze_on_deref=nil, @executor=#<Concurrent::SafeTaskExecutor:0x69ba698e @task=#<Proc:0x7751cd7c@/usr/share/logstash/logstash-core/lib/logstash/instrument/collector.rb:87>, @exception_class=StandardError>, @StopEvent=#<Concurrent::Event:0x6f41e6f3 @set=false, @iteration=0>, @value=false, @copy_on_deref=nil, @dup_on_deref=nil>>>, @options={:polling_interval=>1, :polling_timeout=>60}>, :result=>nil, :exception=>#<Concurrent::TimeoutError: Concurrent::TimeoutError>, :executed_at=>2016-09-20 15:33:34 +0200, :level=>:error}