You want it to be long enough to retain some of the advantages of lifetime appointments. It wasn’t originally framed that way just for fun or convenience, it does have importance.
We also need to make sure they don’t need to go job hunting after their term limit is up, as that would incentivize corruption. They should retain their salary for life.
edit: Reading another comment in here, perhaps its important to note that the main advantage of the lifetime appointment is it allows Justices to be fearless. They can challenge the most powerful people in the entire country, because for their whole lifetime they need nothing more than their current job, which is guaranteed.