Mad_Mat wrote on May 5^th, 2010 at 1:10pm:

its likely that the "average tripper" won't notice a significant decrease in loft/warmth unitl several or many years of use.  Most of the comment I recall from backpacking forums were from thru-hikers on Appalacian Trail or whatever - some of thier comments were more or less that they noticed a definite loss of loft/warmth after about a month of steady use on the trail.

Most likely the loss was due to moisture build-up.  
The true test would be to measure the loft before use.  Measure at the end of the trip.  Wash & dry the bag, don't forget tennis balls to beat out the down clumps, measure again.