From what I understand DWR is simply a coating that helps the water "bead up" so it will fall off the fabric easier. The actual fabric of the jacket is what keeps it waterproof. DWR is simply an extra layer of protection to prevent the fabric from "wetting out." At least, that's how I understand it.
In terms of durability – I wore a Neoshell jacket last year every day during fall-winter. Literally 5-6 months of every day use in NYC. I had an UL down jacket, and would layer the Neoshell over it. This combo kept me warm and dry throughout a very cold, wet winter. I think I washed it once (with Nikiwax Tech Wash), and the DWR coating held up fine. For a down jacket I would just use one of those spray-on treatments so I didn't have to wash it
first of all … unless its one of those known waterproof fabrics like goretex, event, etc … it aint "waterproof"
secondly … once a DWR wears out even on those waterproof fabrics, the water wont roll of anymore, and while the jacket may stay waterproof, the body moisture cant pass outwards anymore … it aint "breathable" anymore .. see why it matters below
thirdly … ive used even dead birds daily, which are considered to have one of the best DWRs in the business, and the DWR will wear out within a year or so of daily use … if you are going to use it then use grangers which have been tested to be the longest lasting reproofer
fourthly … if you wear your down jacket daily, especially the high fill power ones you should wash it more often … the body vapours and oils will degrade the loftiness of the down over time … and every time you wash it it degrades the down abit

as i said before … its not so much external moisture you need to worry about, but internal one generated from you … DWR does nothing to prevent this
from Gordon G. Giesbrecht, U of Manitoba, recognized cold weather conditions experts that trains SAR teams including our local one


http://www.umanitoba.ca/faculties/kinrec/hlhpri/media/Cold_Weather_Clothing.pdf
;)