Dried saliva may be retrieved from a cup, spoon or fork that has been used, a smoked cigarette butt, or a licked stamp or envelope. Feces and urine are not favoured DNA sources. Urine found in a toilet bowl is usually too diluted, though dried urine stains work well. Vomit, on the other hand, can be good source of DNA material, from blood or epithelial cells. Even after a carpet has been shampooed, vomit-borne DNA may still be retrieved from the base of the fibres.
Some samples are better than others.
DNA degrades and breaks down when exposed to heat and humidity, but can last forever if kept dry and reasonably cool. Even at room temperature, a sample can survive for decades. As the quality of the DNA decreases, so does the ability to get a full DNA profile out of it. It might still be possible to get a partial profile, however. Labs currently test for anywhere from ten to fourteen different areas of DNA information, including one that determines gender.