The Collaborative International Dictionary of English v.0.48:

Indefatigableness \In`de*fat"i*ga*ble*ness\, n.
   Indefatigable quality; unweariedness; persistency. --Parnell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indefatigableness
    n 1: tireless determination [syn: indefatigability,
         indefatigableness, tirelessness]