I stopped smacking DS when he was about 2.5 years old, and he began to smack me back. At that age he was still old enough to pick up and place in time out should he need it.
He is now four and has been smacked twice in the last 12 months. It didn't work either time. It just aggravated the situation and made him more angry - and his response was to turn around and smack me.
DD is an angel and hasn't needed smacking.
I am not anti-smacking. I believe you should have the right to smack your children, but so far in my parenting journey I have to say smacking hasn't worked for me.