What is the significance of "seventy times seven"? Twice in the bible, God makes reference to "seventy times seven" (or "seventy-seven times"). Once in Genesis 4:24 (dealing with Cain's punishment for his murder of Abel), and the other in Matthew 18:22 (in an answer to Peter's question of how many times he should forgive a brother or sister that sins against him). Is there any significance to this phrase, and if so, what?
In many ancient cultures, Hebrew included, the number seven often signifies completeness and/or perfection (for more information see either Numerical Sayings in the OT, W. Roth or IVP New Bible Dictionary, ed. Marshall, Miller, Packer, Wiseman, p834). Therefore, it is often used in an emphatic sense. This is seen in Peter's question: "should I forgive seven times?" (possibly thinking he was being a good disciple in making the point that he should always forgive) - Jesus' reply is to be emphatically emphatic! That is, seventy times seven! Jesus often uses hyperbole (overstatement to make a point) in his teaching style, such as the Camel and the eye of a needle in Matthew 19:24.

Lamech's use, again, is to show he is being emphatic. His use of seventy-seven is to make a point.

Hope this helps.

(Edit)

It's also interesting to note that Jesus' contemporary rabbis would teach that a man should forgive a sin 3 times. (See Carson's commentary on Matthew, p.405). This makes Peter's statement of "forgiving an infinite amount of times" even more impressive, though Jesus is not outdone, of course!