If you don’t hit curbs and pot holes, never. I bought my 1991 Mustang with brand new tires/alignment on it in 2010 and had it checked in 2014 when I put new rear tires on it... didn’t need it. In fact the garage didn’t even charge me for checking it.
come to think of it, I’ve never had a front end alignment done on anything I’ve owned, and I’ve replaced tons of front end components....