They are 5L containers and I had 700g of Challenger for 2007.
I find my home-grown hops quite potent - this might be because they are seedless - and I am reducing the amount I use for bittering. However, when using a mixture of Cobb and Challenger I have used ~70-90g for 23L and ~100-120g for 36L. If I was using solely Challenger I would probably use less. Therefore 6+ brews or more again if augmented with some bought hops. For me at least this is more than enough to cover a brewing season, in fact I still have some 2006 hops! BTW, in my experience Challenger does seem to age well.
I totally agree with DaaB - you learn from the previous brew(s) as to how bittering your hops are.
2007 Challenger Hops - Start to Finish in Pictures