http://support.loadimpact.com/knowledgebase/articles/265461-calculating-the-number-of-concurrent-users-to-test
Hourly Sessions x Average Session Duration (in seconds) / 3,600
Simultaneous Visitors = Hourly visitors * Average Time Spent on Site (in seconds) / 3600
=8000*(27*60)/3600=3600 cocurrent users persecond for ds