When should the hadd punishment for zina be carried out on a man?

The hadd
punishment for zina is incurred when the head of the penis enters the
vagina. In that case the two circumcised parts have met, i.e., the site of
the man’s circumcision and the site of circumcision in the female. If
penetration takes place, then this is the zina which incurs the hadd
punishment, whether the man ejaculates or not, or if he penetrates to this
extent then ejaculates outside, and whether his penis is erect or not. 

It says in
al-Mawsoo’ah al-Fiqhiyyah (24/23), explaining the conditions of the
hadd punishment for zina on which the fuqaha’ are agreed: There is no
difference of opinion among the fuqaha’ that in order for the hadd
punishment to be incurred, the head of the penis has to disappear into the
vagina. If it does not enter it at all or only part of it enters, then there
is no hadd punishment because that is not intercourse. There is no
stipulation that ejaculation must take place or that the penis must be erect
at the time of penetration. The hadd punishment must be carried out whether
he ejaculated or not, and whether his penis was erect or not. End quote. 

Secondly: 

This does
not mean that a person may be negligent about committing haraam actions so
long as they do not reach the level of zina! Rather the intention here is
simply to define the zina that incurs the hadd punishment. Being alone with
a non-mahram woman, touching her or kissing her are all undoubtedly haraam,
and the Muslim must avoid such actions for fear of the punishment of Allaah
sooner or later, in this world or in the Hereafter. 

We ask
Allaah to guide us and make us pious and chaste. 

And Allaah
knows best.