According to a recent report,

...the percentage of workers who had employment-based health coverage from their own job in 2012 was: • 12.8% of workers who were employed less than 30 hours per week • 33.6% of workers who were employed between 30–39 hours per week • 60.5% of workers who were employed at least 40 hours per week Source: "Trends in Health Coverage for Part-time Workers," Employee Benefit Research Institute Press Release, May 22, 2014, http://www.ebri.org/pdf/PR1078.Prt-time.22May14.pdf