BodhiSeed
I'll hand the querent the timing deck, get them to shuffle it briefly, then cut it at the point where the deck wants to separate (rarely the middle). I'll then look at that card.
Jera, from what Nisaba said, it sounds like you would only draw one card. If you draw a Four of Swords, that would be 4 weeks or 28 days. My guess is that this method will give you a close approximation, rather than the exact amount of time (ie. 26 days).