Most people would consider someone part time if they work less than 40 hours weekly. Some professions require huge hour commitments so part time work there could be a larger number. Legally, the affordable care act considers someone part time under 30 hr/wk per week and as far as the IRS is concerned you are part time if you work under 25 hr/wk, or under 35 hr/wk if other employees work substantially more (Income Tax Regulations 1.105-11(c)(2)(iii)(C).