I started smoking at age 21, when I graduated college a year early, and smoked nonstop after graduation until age 23.
At that point, my life basically fell apart, and it was definitely related to being high 24/7.
So I quit cold turkey. By age 25, I had started my own web business that was very successful. SaaS business, recurring revenue, amazing growth, etc. I was driving a fancy car, living in Palo Alto, and was on track to being a millionaire by my late 20's, almost guaranteed, just based on the growth of the business. I was so financially secure that I even got married and had a kid very young.
Of course, running the business (plus starting a business at the same time) was stressful. There were customers, partners, employees, etc., to deal with. And a toddler, and new wife.
I had very little support or mentors. None of my family or friends were entrepreneurs and I had no connections.
Eventually, I started getting migraine headaches on a regular basis. Around age 27. I convinced myself that smoking marijuana again was a logical thing to do, despite the fact that it had literally destroyed my life after college, and I had to rebuild my life back up again already.
I was smoking large quantities of high grade stuff. I had a lot of money from my business, so I went all out. Oils, kiefs, fancy bongs, the whole deal.
Being high all day, I started making terrible decisions and being really reckless with my business.
Long story short: within 12 months I went from a highly profitable, debt-free, lifestyle business that allowed me to effortlessly live in the heart of Silicon Valley (out of choice -- I could have lived anywhere I wanted to), to having a nervous breakdown, bankrupting my business, and ending up $250,000 in debt.
Today, I am 34. I am still paying off the debt from that mistake. I have no business, and work a "shitty" (meaning, comfortable six figures, but no real ownership) job as a software engineer. I am regularly depressed when I think about what I lost, and how foolish I was to lose it.
This is an extreme case, and of course only an anecdote. But it's pretty black and white: smoking a lot of marijuana all the time made me dumb and caused me to make really bad decisions.
As for weaning off of marijuana and its resultant impact on the sharpness of mind:
I found that after 3 days, there is a very noticeable difference. You are probably about 70-90% recovered at that point. But it takes a good 6-12 months to get that back that really sharp "edge" that you once had, the remaining 10-30%.
But, as I hope I have related in this story, it's not just about getting back the intelligence and sharpness of mind. It's that even when you get that stuff back, you will still be left with the consequences of whatever dumb shit you did while living your life high all the time.