How tall should tomato cages be?

Depends on the tomato variety. Indeterminates keep growing until you kill them or a freeze does.

I mostly plant Goliath and I've had them grow to over 10 ft. I use 8ft fence posts to stake them.

If I were to make cages, I would have them no shorter than 6ft.

I've got one better boy hybrid and one super sweet 100 cherry tomato

I drive my stakes down to eye level so a little under 6ft