When is the next update?
The developers decide on a set of goals that will define the next release. Some of these goals (usually bugfixes) are mandatory, and the release will not occur until they are fixed. Some (usually features) are just goals, and may or may not show up in the release they are targeted for.
You might find some specific comments about the next release on the Next version page.
Although we don't set specific dates for future releases ahead of time, once we have some good features ready and nothing too unfinished, at somepoint it will be collectively decided that the release should be real soon, and from then on everyone should focus on polishing up the release within the following weeks. A feature freeze might be declared to prevent last-minute additions which could add bugs. Typically a release candidate is then declared on the forums about a week before the release.
Previously, you could get an idea of how far away the next release is by viewing the latest release tracking bug, a system not used anymore.