Not serious at all. Speaking as someone who has drunk coffee every day for decades, the worst that happens when you don't get your coffee fix is a persistent headache - that ends within ten minutes of drinking coffee.
Then it was all gone. Not too serious. But, the headache you're describing is just the start of withdrawing.
The literature suggests 4-14 days for withdrawal iirc. Less sure about the lower bound.
Still, I'm glad I did it - it was definitely an unhealthy addiction for me. I'm now 15 or so years mostly caffeine-free.
It seems there is a gene that determines whether someone is a fast or slow metabolizer of caffeine. If I ever get such a test done I'll definitely check if I'm on the slow spectrum. It's associated with worse coffee outcomes.
https://well.blogs.nytimes.com/2016/07/12/for-coffee-drinker...