The statutes regarding this are set by each state, so there's no simple answer. Generally, however, the employer must comply with the order on the next payroll date. The order itself, a copy of which is usually provided to the employee either by the state or by the employer (again, depending on the state regulations), should specify when the garnishment must begin. There is generally a provision that allows an employer to wait until the following payroll if the order is received just before the payroll date, so they don't have to re-do their payroll. For instance, the order might specify that the garnishment must begin on the first regular paydate falling 5 days or more after the order is received. (The orders are often sent certified with return receipt requested, so that the state knows when they were received.) There may be other details that might allow an employer to delay complying with the order, but if so they will be indicated in the order itself. Most states have heavy penalties for employers who don't honor such orders. Also, I think all states now have "new hire reporting" laws, which require employers to report all new hires to the state angency that enforces child support orders. The states all cooperate with each other in collecting child support. Before the advent of computers, it used to be that someone could largely avoid paying child support simply by changing jobs or moving to another state any time a support order caught up with them, which could take quite a while (or might never happen at all). These days it's a lot tougher. In recent years I've gotten support orders from other states within 30-60 days of someone being hired.

In Alabama, an employer has 30 days to start garnishing and must send garnished wages at least once each 30 days. Strictly speaking, a garnishment can be used for back child support. A wage withholding order is used for current child support. A wage withholding order has different rules than a garnishment.