There are annual and perennial species of antirrhnum Gretel majus is a annual, I have Braun-blanquetii which is a perennial so really depends on the variety you're growing, most of them do seed around though so you'll find babies popping up.

A. majus could be classed as a short-lived, tender perennial, but in my experience, the first year's flowers are always the best.

They do sometimes last through to flower a second year but I wouldn't rely on them. Also, they seem to get rust in the second year so best to start again each year. They also seed around gently.