Realize I''m a newby and don''t have the benefit of a good longitudinal view.
However, it seems that several factors are driving the market- which include-
-Aging of Baby boom generation- as they are middle aged and beyond many who dabbled or collected in their youth are returning in a more serious manner as they have more time and money for hobbies as their children leave home, they reach retirement or peak earning years, etc
-State quarter program has kindled all sorts of interest in young and old
-ebay, etc enable folks to view and think about coins on a whim 24/7
-slabbed coins make folks more comfotable with sight unseen purchases
-lots of other stuff ;)
-Coin market seems to go in phases of interest/retreat.
So, I'd guess it will continue for awhile, at least, as more boomers return, more folks get comfortable with online shopping (if shady prctices don't kill interest of the unwary/naive), then peak and retreat abit as sq program ends and boomers get too old to see, even with loupes ;

Market has increased so much in the last several years that a correction will come as folks get priced out of certain markets and sets, in my guestimation. A wild card is what precious metals do, of course- two ways to look at that. ND has a nice analysis of why gold may retreat on the board, but as China and India get wealtheir (both are countires where lots of folks like to own gold, historically- in India as jewelry) that may offset some of the forces ND discusses re strength of the US dollar.
Don
So this is my longwinded way of guessing another 4 or so years.