Nicotine is out of your body in 3 days, the half life of nicotine is about 2 hours. Its off product Cotinine has a half life of 24 hours. 93% of nicotine is processed out of your body in 8 hours! The Cotinine takes 4 days to reach 93%. If I'm not mistaken, blood tests for insurance look for Cotinine and not nicotine, which can be in your system for up to a week.
Symptoms last longer, but generally peak with the physical withdrawl at 3 days. However, at approximately 3 weeks, your brain begins to reduce nicotine receptors back to normal, so you experience another physical change, I didn't feel a negative effect out of the normal though.
I argue it like this, the physical withdrawl last 3 days; you continue to crave and urge after that because of triggers. If you sit on the couch and pick up your phone every time, the first time you do that without your phone, and probably several times after that, you will go to look at your phone. It is, very much so, a pavlovian response. Smoking is no different. The problem is, it takes a month to months to undo the triggers as they are so wide spread.
Now, I would argue you made it through the most difficult part of quitting and have proven you can do it. Quitting is different for everyone, so what worked for me or didn't may be the opposite for you. I concentrated on important days and times. I knew if I could make it to day 3, I would make it to 1 month.
You, if you haven't already, will make it past day 4, because you know what led you to go back (besides the availbility); overcome that!
