And in case anyone was wondering about the easiest way to figure that out: https://www.google.com/search?q=4.5+million+minutes+in+years

Yes, but he wasn't working continuously for 8.5 years. If a full time work-year is about 240 hours, that's 312 1/2 years! I admire his persistence.

240 hours is a very short full time work-year.

(52 weeks/year - 2 weeks vacation) * 40 hours/week == 2000 hours. Many workers exceed that.

50 weeks/year * 40 hours/week * 60 min/hour = 120000 work min/year -> 37.5 years.

So, most possibly, Watterson was refering to the total time, not just the time spent at work

365.25 days/year * 24 hrs/day * 60 min/hr = 525960 min/yr -> 8.5 years which is STILL a very long time.

I have not (yet) had to spend so many years on something I did not like.