If Moore's Law sort of performance is obtained for Flash products, and we take the 4GB Nano as the benchmark of what's feasible today, I'd say... 18 mos * 8 = 144 months = 12 years....
But more than that... I think you're not going to see anything more than a niche market cater to that. If you look at digital music players, understanding that there have been audiophiles clambering for increased bitrate for some time, the majority of the rest of us don't care, and the data backs that up. The iPod franchise has *never* gone after a bitrate increase in the five or six years it's been around, has it? And the store sells more and more digital music at 128 without a significant number of complaints regarding quality...
So I understand the audiophile's perspective. I really do. But I think that the determining factor in seeing 300GB iPods any sooner than that is going to have to be based on some other killer app that needs that space, and not lossless audio, because the market just doesn't support the latter application in volume. Even if you and thirty other MR users who are audiophiles and insist on lossless encoding reply to this post saying, "I would buy one in an instant," it's a big step from there to the millions of iPods Apple sells every quarter... the majority of which go to people who don't care.
So I guess if you want to see the size keep marching up quickly, we need to see adoption of those other technologies for the iPod. For instance, it doesn't hold *that* much video. Not nearly as many people can take the iPod music approach to video today, and take every video they've ever owned, and rip it into their iPod, and put all the DVDs and so on away. So right now, the success of video on iPods is *the* driving factor to see bigger iPods, I think.