Currently, users have a 140 character limit for both tweets and direct messages, but in July Twitter will be removing that limitation from direct messages. The company has announced the new change, along with an updated API for developers. By announcing it early, and providing documentation for developers, the company hopes that when they launch the feature in July all apps will be ready to support it.
As for the launch, it is not noted exactly when the new functionality will go live. Developers can begin adding the support now, and Twitter will be providing an update with how developers can begin testing for it.