Most states: 5-7 years
Most US states treat unpaid parking citations as civil debts subject to the general statute of limitations on government claims — usually 5-7 years from the date of citation. After that the city cannot file a new action to collect.
Exception 1 — judgment was entered
If the city already obtained a court judgment for the citation (because you missed a hearing or were defaulted), the judgment has its own statute (usually 10-20 years) and the citation amount is enforceable until the judgment expires or is renewed.
Exception 2 — DMV registration hold
Many cities place a registration hold with the state DMV for unpaid citations. The hold can persist indefinitely until paid, regardless of the underlying statute of limitations on the debt. To clear it, you typically must pay or have the citation administratively voided.