How long will scotch keep after its been opened?

I recently ordered a 1.75L bottle of Johnnie Walker Gold Label from grandwinecellar.com.

Unfortunately, I was unaware that Scotch could actually go bad! Anyways, my question is: How do I prevent it from doing so? I'm not a frequent drinker so though I'd open the bottle right away (I'm SO curious to see how it tastes) I doubt I'd go through the bottle fast enough to outrun the oxidation (especially once I reach the 2/3 or 1/2 marks). I really don't want it to start losing flavor on me either.

I heard that dropping clean marbles into the bottle is a good way to prevent oxidation. What do you guys think?

[blenderm]

I'm sure that there'll be a lot of answers for this but don't use marbles as they do nothing and may contaminate the whisky. I don't know where you heard that it goes bad as that's a myth, what happens is that there is a slight lessening of the flavour intensity, always assuming that you seal the bottle after you've taken a drink. Normally a bottle will last up to a year without a noticeable drop in flavour and if it's kept out of sunlight and away from heat could last much longer.
Just forget all the myths and enjoy it
Slainthe