This is a good question. It may be a while. For the 5K iMac, Apple was able to develop a custom solution for driving the 5K display panel.
For an external 5K monitor, Apple will likely need to implement the
Display Port 1.3 standard. That requires computers with Intel's Skylake processors. Those aren't supposed to arrive until late 2015 or early 2016.
Thus, it could be quite a wait for a new 5K Apple monitor. Theoretically, the company could release a 4K monitor or a 5K monitor that uses two DisplayPort 1.2 cables but that doesn't seem too likely at this point.