Federal law does not require that employers allow their workers time off to vote, but the majority of states have at least some level of protection for employees who want to leave work to engage in their civic duty. The specifics vary by state, but in each state, the rules apply to almost every type of workplace. Employers are required to know the laws and to provide adequate accommodations according to the rules of that state. Though many states allow employees to have up to three hours off during the time the polls are open (the number of hours varies by state), nearly all of the states allow employers to refuse time off to vote.
Check this resource compiled by the AFL-CIO for a table of each state and their respective laws surrounding employee time off to vote.