Back to the topic just to tell you my experience.
Long before discovering Audax, I understood how powerful the effect of caffeine withdrawal can be because I took the habit of drinking many (about eight) cups of coffee on working days, and just one or two on Saturday and Sunday. Saturday afternoon headache and drowsiness regularly kicked in. Saturday night I usually was a zombie. Sunday was a little better. At first, I attributed it to necktie and hard-collar shirts on working days. Then, I noticed that on Monday evening I was particularly awake and eventually I had a light bulb moment and understood it was all due to irregular assumption of caffeine.
In my personal experience, halving the daily caffeine assumption in a week's time (i.e first week gradually pass from 8 to 4 cups/day, second week 4 to 2, etc.) is a reasonably fast decrease, in the measure that it doesn't give me negative side effetcs. Of course I'll have no difficulties in going to bed early, while it might be a bit of an effort if I have to stay awake later than usual.
Cutting away the last daily cup of coffee, the early morning one, is by far the hardest part of the "decaffeination process". But once I pass a couple of weeks without drinking any coffee (nor tea, nor anything else with caffeine), provided I haven't accumulated any sleep debt right before the start, the first audax night passes away without any sleep and any effort. I usually wait until sunrise to drink a coffee and then... boom! Even if I've been riding for 24 hours, after one cup of coffee I'm completely awake. I then can go on for the whole day, totalling 36 hours of riding, without great effort, I just drink another cup of coffee or a can of coke if I start feeling drowsy, but not in the late afternoon, so that when I finally come to a rest caffeine is going away and I can take full advantage from my sleep.