Look at this paragraph a couple above the one you quoted:
> In the mid-Nineties, in my mid-twenties when I was working as a journalist in London, I took enough recreational drugs to keep me awake for nine days, at the end of which I was psychotic, sectioned, sedated and held in hospital for four months. That might sound dramatic, but I did it to myself and for all I know the treatment (including drugs since withdrawn from use) and the incarceration saved my life.
So, mid-90s, freaked out on recreational drugs, ended up in the hospital for four months.
Went on legal brain-altering drugs for a decade. Which would have him going off of them somewhere in the 00s.
...and then if you look at the paragraph right above the one you quoted...
> It seemed clear to me that if I could reach such an altered state through intoxication and insomnia once, and then do it again 20 years later through silence and concentration [...]
...we can pencil in about a decade of being off the legal brain-altering chemicals. Whatever triggered this, it wasn't the result of going cold turkey off of something he'd been taking up to the last day before starting the meditation course.