This will happen gradually as a consensus for how the different new properties should behave emerges. Webkit, for example, recently started listening to border-radius without the prefix. There is no clear timeframe and neither can there be. It’s done when it’s done.
(I actually hope that browser vendors won’t be too eager to drop prefixes. This would be a bad idea as long as different rendering engines still behave differently. It’s important that there is consensus, otherwise and without the prefixes we will have to resort to hacks like in the dark past.)