Although it is scientifically proven that human scent can persist as a trail for weeks and even months, it is another question, if this also applies to human scent samples captured on different media and stored in different environments.
Collect on cotton medium
In 2005 a study showed that scent samples that are captured with sterile cotton gauze and stored in sealed glass vials, the individual human-scent compound combinations still can be measured by a gas chromatograph after nearly three months.
Store in glas vials and avoid direct sunlight
Studies showed that cotton proved to be the best known media to capture and store human scent for a long period of time (except for frozen samples). Furthermore glass has been determined to be the optimal type of storage container for human scent samples as the cotton materials stored in that manner had less overall compounds contributed through the storage method, but also significantly less compounds that have been previously reported to be present in human scent. Glass storage materials are also the storage container used most readily by human scent canine units across Europe. Further results show that scent samples should not be exposed to excessive amounts of UVA ⁄ UVB light. (Hudson 2009)
Best results with fresh samples but aged samples can be a considerable alternative
In another study in 2005 it has been shown that stored scent samples degrade the most in the first two weeks (to a level of a correct recognition by a dog in every second test while having more than two samples for choice). After that the samples degradation stopped and the recognition rate reached a steady state at a lower level. (Schoon 2005) However: studies conducted by the FBI in 2004 showed that scent samples collected after two weeks from an object were reliable enough so that trained mantrailers were able to correctly identify the corresponding individual with a success rate of 70-80% (directly depending on the level of real case experience of the teams) (Stockham 2004)
(Literature references collected from: Untersuchung des Einflusses der Alterung menschlicher Geruchsspuren auf die Ausarbeitung der Fährten durch Personensuchhunde, Angelika Wolf)