From what I get, the nicotine part itself dosent hang around in the body for that long, its half life is only about 0.5–3 hours, so most of it gets processed pretty fast. Usually, the thing that tests focus on is cotinine, which is basically the main metabolite of nicotine and it sticks around longer, like noticeably more.
In a general sense, nicotine tends to clear from the bloodstream within a day , give or take, but that can swing a lot depending on how much and how often someone uses nicotine. Cotinine can remain detectable for several days, and it can be found in blood, urine, and saliva. In practice, urine can sometimes show a longer , more sensitive detection window compared with blood, because the cotinine levels there are often higher.
Hair is kind of different, because nicotine plus cotinine can get built into new hair growth, so hair testing might reflect exposure over a much longer stretch of time than blood or urine.
So, the simple version is: nicotine leaves fairly quickly, but the fallout compounds , especially cotinine, can still be there for several days or even longer depending on the individual and the test type. There is not one single exact detection period that fits everyone perfectly.