May I ask what your dosing was? I honestly don't doubt you one bit - but as an ex-25-a-day smoker, a week of endless pacing and sleepless nights and hiding in my cupboard, followed by a week of sleeping and grumbling, and a subsequent year of basically no alcohol (triggering), there's just no comparison to a week of headaches and thirst (caffeine). But, I suppose I do say this as someone with coffee in hand, so maybe staying quit with caffeine has been that much harder...
For caffeine, it’s hard to say, but probably at least a liter a day of Coca Cola or similar. For cigarettes, peak usage of half a pack a day. Quit both cold turkey (many years apart). Quitting soda gave me pretty significant headaches for about a week. Quitting cigarettes had no noticeable side effects other than cravings and some antsiness every hour or so. Note also that I have since relapsed on both, but was cold turkey for at least 6 months.
Quitting coca cola is different from quitting caffeine though. You are quitting sugar along with caffeine.
I definitely didn’t quit sugar, although probably reduced it significantly. I was still drinking juice pretty regularly. I’m pretty sure the headaches were primarily from caffeine withdrawal.
A slow taper off of caffeine is much more comfortable, even if it does take longer. Can't really speak to the nicotine side.