My guess is that it will be released with the next generation of Nexus, so probably some time in the Autumn. Other manufacturers will start to roll it out a few months later.
As Kate says, it's up to the manufacturer which devices they provide it for. Recent high-end devices probably will, the older or lower end you go the less likely. Different manufacturers are better or worse for this too. Finally, if you buy a device through a carrier you need to wait for _them_ to stop dragging their feet (often adds months to the process).