An alignment is strongly recommended when you replace all 4 tires for best wear and tread life.

When suspension parts are replaced or if you hit something (like a big pot hole). Or more often just to be sure.

You should never need an alignment.

I like to align and rotate my tires every 4,000 miles for the best treadwear. Also check your tire pressure every other week or so.

A lot of websites recommend an alignment every year or two. Instead, I do it with each new set of tires

To always be in factory spec, twice a year. To barely go outside of factory spec, annually. To never see poor tire wear, every 2 years. To fix it when your tires are already shot, every time you swap tires.

It depends on where you live. Around here it’s safe to say if you leave a shop with perfect alignment, you’ll be out in a month. I know some people live in areas without craters and bumps the solize of asteroid impacts every 3’ though, so the answer is truly “anywhere between every seasonal tire change over to just when new tires are installed”

Twice a year or once every 6 years. Depends on your roads and driving style. No right or wrong answer.