How long can a curse be in operation
 

God very clearly reveals in Exo 20:5 – thou shalt not bow thyself to them, nor serve them because I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the 3rd and 4th generation of them that hate me:

Numbers 14:18
The Lord is longsuffering, and of great mercy, forgiving iniquity and transgression, and by no means clearing the guilty, visiting the iniquity of the fathers upon the children unto the third and fourth generation.

(GNB) 'I, the LORD, am not easily angered, and I show great love and faithfulness and forgive sin and rebellion. Yet I will not fail to punish children and grandchildren to the third and fourth generation for the sins of their parents.'

The above verses clearly indicate that a curse can be in operation upto the 3rd and 4th generation which is equal to 140 years