Jesus's atonement equivalent to sinners' punishments When Jesus went to the cross, he experienced 


the ordeal of his arrest
agony for three or so days
The loss of the rest of his natural life


When a sinner sins (without accepting Jesus), he experiences a long time in Hell.

How can Jesus be said to take on the sins of mankind, if the punishments of those sins of mankind exceed the suffering Jesus actually experienced. Why would Jesus not have to personally go to Hell in order to take on the punishment of the sinners?
Since you did not ask for the doctrinal position from any particular denomination, I will answer this question from a biblical perspective.  The Bible does not explain how Christ's sacrifice was sufficient, but it does make clear that it was sufficient.  For instance:

Is 53:10-11 (NASB)


  But the Lord was pleasedTo crush Him, putting Him to grief;If He would render Himself as a guilt offering,He will see His offspring,He will prolong His days,And the good pleasure of the Lord will prosper in His hand.As a result of the anguish of His soul,He will see it and be satisfied;By His knowledge the Righteous One,My Servant, will justify the many,As He will bear their iniquities.


Since the Bible is not clear as to why Jesus' sacrifice was sufficient, believing that it was sufficient is a matter of faith.  There are some helpful explanations which have been given, but they are speculative:


Jesus was Innocent - When an innocent person accepts the punishment for someone else, it softens the wrath of the Judge and the sentenced may be reduced.
Jesus Became Sin - According to 2 Cor 5:21, Jesus became sin for us.  Many people interpret this to mean that Jesus became sinful and was spiritually separated from His Father.  He experienced spiritual death and a loss of communion with the Father.  This change is beyond any specific punishments which might be experienced.
Jesus is Divine - As a member of the Godhead, Jesus' sacrifice is more significant than that of any mere created being.