Due to vast amounts of equipment still running on 5V, and needing maintenance, I would imagine quite a long time yet.
For new designs, things are a little different. Personally, if all the parts I need in a design are available in 3.3V or whatever, I will happily use it and forget 5V.
The question may be analogous to "how long will DIP parts last, before being replaced by SMD?" (Edit... He He, I missed the DIP part of your post when I wrote all this and only saw the 5V, must read more carefully!)
With both questions, there are no reasons not to use lower voltage and SMD parts in new designs, and many reasons *to* use them. Including the fact that many newer parts are only made in 3.3V and SMD.
I would imagine that prices of 5V and DIP parts may well start to rise, as manufacturers produce less of them and more low voltage parts.
Another option of course is to produce parts that can run at 5V *and* 3.3V without complaining.
Just my thoughts.
FoxyRick.