Back in my working days, I worked full-time and that was 35 hours per week. Then, in 1993 the standard work week was increased to 37.5 hours without any pay increase for salaried employees.
When I switched to working part-time in 2001, I reduced my weekly hours worked to 20. Many of my benefits were reduced, too, including the employer subsidy for health insurance. I had to pay 50% of the premiums instead of 25%. I don't know what the cutoff was to no longer be considered full-time. Working 20 hours per week was the minimum to remain eligible for group health insurance.
In 2007, I reduced my weekly hours from 20 to 12. This made me ineligible for group health insurance. I offered to pay 100% of the premiums, something I thought would be a no-brainer for the company to accept. But they turned me down, saying it was not "fiscally prudent" to include me because I had suddenly become "high-risk." I was now in a small group of employees, mostly those near age 65 (I was 45 at the time) who worked less than 20 hours per week. I was pretty annoyed. I also lost nearly all the rest of my few benefits. But it assured me of ER by the end of 2008 so it all worked out just fine.