I don't really think there is a normal. There seems to be a lot of variation. I checked the gf's CTT at about 10k kms and it had about 6mm left but it's pretty well not budged the last two times I've checked, at 36k kms it still had about 5mm left, so I put on a Versy anyway and left it on the first notch, so I don't have to worry. It's at 41k kms and it still sounds normal, but I'll be pulling it off at 42k kms and I would not be surprised if it's still got something left.
With regards to the gasket, it's not copper it's an aluminium crush gasket, you'll need to get a little hook from an autorparts store to make it easy to get at. Believe me it's the best $3 you'll spend. Try to dig the point of the tool into the middle of the gasket so as not to scratch the mating surface of the engine. It will all be blackened so use a strong torch to see where the edges are.