I've proved that myth to be false myself - Nobody can take just one darvocet!
However, you might take a look at http://www.rxlist.com/cgi/generic/propoxnap_cp.htm
Depends on the drug test method:
Urine - 2 days
Hair - up to 90 days
Blood - less than 24 hours
More detailed info below:
http://en.wikipedia.org/wiki/Drug_test
If they are testing for opiates, it wont show up on that panel. they need to test for propoxephene I think exclusively. but Im not positive. maybe someone else will agree with me. or not.