I suppose that's a matter of how good your password is,
and how exposed the box is to potential attackers ...
E.g which services are open to the world, and how they're
set up ...
It's possible, not even overly hard, to rename root (for a time
[just for the heck of it] mine was called ruth) ... just edit
/etc/passwd and /etc/shadow and do your thing.
The thing is, however, that the problem is the quality of
ALL your passwords ... I have many login-attempts for other
users (randomly chosen names); once the attacker has
ANY access he can (necessarily) look at /etc/passwd
and try on knowing root's real name ... so you need "safe"
usernames for ALL users, and sensible passwords for all