In my wildest dreams, 10 years at least; possibly 20 depending on how good the projections in terms of IPv4 exhaustion are.
Frankly, it's not about IPv4 exhaustion, it is about market adoption of IPv6.
IPv4 address exhaustion will never occur. As we approach 100% allocation (being now a tad over 60% allocation), the level of administrative pushback on a new allocation requests will increase, until they get to a point where ISPs will find it simpler and more cost-effective to purchase allocations from each other. A market will develop, and the price of an allocation will change as demand approaches supply. Eventually, the price of purchasing an allocation from another ISP will be perceived and used as a competitive weapon, and the market will become a very difficult one. But supply will never become zero. It will simply become expensive.
That is economics 101.
We will, at the same time, go through an S curve in IPv6 deployment. We currently see a certain level of ISP and Enterprise deployment, and sometimes see edge-ISP-edge connectivity, most commonly on research networks. At the point where it becomes probable that the only or preferred way one can access a peer enterprise with which one has or seeks a business relationship is via IPv6, perhaps for some critical application, IPv6 deployment will start to grow very strongly. As it approaches saturation of the current IPv4 Internet, systems which choose IPv6 in preference to IPv4 will use IPv6 when they can, and IPv4 traffic will start to subside. When IPv4 volume becomes too marginal, ISPs will start shutting it down.
The question(s) that raises are "will <pick a point in that game plan> ever occur, and if so, when?"
My crystal ball is as cloudy as anyone's. But I would expect that it is all a matter of economics.
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