According to Official Guinness Records,
The longest performance by a solo artist lasted 501 hours and was achieved by Kuzhalmannam Ramakrishnan (India) at the Rhythm Therapy Hall, Nandavanam Hospital, Ottapalam, Kerala, India, on 5-26 July 2009
This record is the overall category for music marathons - Kuzhalmannam Ramakrishnan also holds the record for longest marathon hand drumming.
For a complete list of 2009 records, please visit 2009 Guinness Records in India.