BOINC Software Update Frequency
From Unofficial BOINC Wiki
 BOINC Server Update Frequency
Well, there is nothing that you have to do about this. This software is under the control of the Project and they will update it when the have a need and the time to take the system off-line to make the updates. Right now the software is in an evolving state (2005) and this is expected to last at least another year. Sometime there after, we should see more stability in the BOINC Server Software.
One item to keep in mind, not all of the Projects will be updating their software at the same time. So it is entirely possible that you will encounter several versions of the software as you move about the Projects. This can be especially noticeable when using the Project's Web Site.
 Science Application Update Frequency
Science Applications will be updated on an as needed basis. However a Participant has no need to be concerned about this. The BOINC Client Software will automatically download and use new Versions of the Science Applications as needed.
Participants should also not be concerned if there is a new Science Application downloaded but not used immediately. Normally Work Units will be processed with the Version of the Science Application that they were originally assigned with.
Each Work Unit issued to you will be processed by a specific version of the Science Application. When those Work Units that you have already downloaded have been processed the obsolete Science Application will be removed.
All new Work Units issued will be processed with the new version of the Science Application. You will see the version of the Science Application that will be used to process all of the Work Units currently buffered by the BOINC Daemon in the "Work Tab" of the BOINC Manager.
It is possible for the Project to change the version update so that all existing Work Units will be processed by the new Science Application. This is not going be the normal process however. In almost all cases, work to be processed by a designated version of Science Application will be processed by that Version.
 BOINC Client Update Frequency
The BOINC Client Software will be updated on an as-needed basis. Once the program has most of the essential features it is likely that the update interval will be on an semi-annual to an annual basis. As part of this there will be additions to add features that become desired and a determination that there is value to include the change. With the project being Open Source many people can work on continual refinement and with this larger pool of talent it is likely that we will see many nifty and useful features added in to the basic software.
At the moment, however, it is happening on a much more frequent basis. There is sharp debate on the Forums about the quality of some of the changes. For my own part, I have not seen many of the problems that other Participants have seen. That being said, Your Mileage May Vary.
 Required BOINC Client Update
There are going to be some updates to the BOINC Client Software that you will have to install. Most likely, updates to the BOINC Client Software are not going to be mandatory for all Participants each and every time.
Each of the BOINC Powered Projects will set the minimum revision of the BOINC Client Software that they will allow. So if you are participating in two Projects, one may require an update and the other does not require that update. In almost all cases the latest Version will work with Projects that require an earlier Version.
On the other hand, the changes in the BOINC Client Software are likely going to make the participation in projects more fun and give you additional features that will make participating in one or more Projects more enjoyable.
A specific example is at this time, the Einstein@Home Project and the SETI@Home Project are requiring Participants to use a BOINC Client Software Version of 4.19 or later where the rest of the Projects (at this same time) will allow the Participants to use an older Version, as low as 4.04 for some Projects. This is because the Einstein@Home Project uncovered some new issues in the software that were corrected in the later Versions. Since not all Projects were equally affected by these problems some feel less pressure to require Participants to upgrade.
In the cases where a Project will require a more up-to-date version of the BOINC Client Software you may see the message: "Message from server: No work available (your core client is out of date - please upgrade)", which, of course, says that you should download and install the upgraded BOINC Client Software or you will not get new work from this Project.