With me the brain fog lasted I would say a couple of weeks and was still there a little for a week or so after that. I have heard people say it could be that your blood-sugar level is lower than when you smoked and that drinking fruit juice can help boost your leves. Not sure if that actually works or not. General crankiness and having a short fuse was probably the longest lasting symptom for me. That lasted off and on for a good 6 months I would say. Now, that wasn't constant or all the time but it did show itself from time to time. I think that was just me being impatient with the whole adjusting to being a non-smoker process. That symptom can vary hugely based on what your personality is to begin with I think. Many say "embrace the suck"and "focus on the positives" which is very true in reality but not everyone is able to do that. It does help though to step back from time to time, take a few deep breaths and think about the big picture - your end goal of having a healthier life and one free from being a slave to your addiction. Those are pretty big prizes so it''s no wonder there's some ugliness involved.
Keep going and congrats one your 1st week! There's the worst one done and in the bin!