Depends, if it's self evident in the context what it means, like in a method which is declared as (p *player) Attack, p makes perfect sense (to me) rather than typing 'player' everywhere in the function, just like typing i instead of index.

In short, I don't think there is a particular rule that applies, that said my impression is that a lot of variable names in Go code are typically three letters or at least two, like err, buf, src, dst, ok etc.