How many KDF rounds for an SSH key? What is the best practice on choosing how many key derivation function (KDF) rounds/iterations when generating an SSH key pair with ssh-keygen? 

Am I correct in saying that it is unnecessary if the passphrase is strong enough? Is there some kind of formula for how long it would take to crack a private key given the passphrase length and KDF rounds?

My guess is that the default value of 16 for when generating an Ed25519 key is small enough to be almost imperceptible for normal use (logging into a server from the command line), but adds significant expense to any brute-force attempts. I'm wondering if it's worth increasing it.
I did also tried to find a good value for the -a flag, in a MacBook Pro Mid14 (i7), trying to login in to a Debian 8.5, I had this results:


-a 1000 Took about ~20s.
-a 100 Varies between ~3.5s to ~6s.
-a 64 (4x default value) ~3 to ~9s.
-a 16 (default) ~2s ~2.5s.


In the end I just stick with the default since I'm not a paranoiac and I don't like to wait, however I did found in my research a value of 100 is not uncommon to choose.