According to Official Guinness Records,
The longest time in an abdominal plank position carrying a 60-lb pack is 17 min and 26 sec and was achieved by Eva Bulzomi (USA) at Axiom Fitness in Boise, Idaho, USA, on 6 July 2013.
For a complete list of 2013 records, please visit 2013 Guinness Records in United States.