seven times the years...

[mallondwen]

The Voice of Saruman
'The Watchwood we will call it. Not a squirrel will go here, but I shall know of it. Leave it to the Ents! Until seven times the years in which he tormented us have passed, we shall not tire of watching him [Saruman].

ok, I am wondering, why 'seven times the years in which he tormented us?' I mean, I know that Saruman had been cutting down the trees for about eight months by then, cause he started about the time that he imprisoned Gandalf which was in July, it was now March. It hasn't even been a year yet. So I guess my question is: Is this just a metaphor Treebeard is using to imply 'a very long time' or, is it literal? If so, why this time period, 'seven times the years in which he tormented us'...?

Any thoughts are most welcome.