Unfortunately I do not have a timeline estimate on it. I am working as fast as I can on the things I see as most important to drive the popularity of the project and to get enough revenue coming in from the store to sustain me. Once the revenue is consistently providing enough to sustain me I will have more freedom about what I want to work on.
The history of SiteOffice is that I implemented the current version using ExtJs, it took a lot of effort to get the main layout and plugin system implemented and then ExtJs changed their license to GPL (not compatible) which means I can no longer get upgrades and therefore I do not wish to move forward with the ExtJs version of Site Office. The current version is a working first draft of a plugin system for application style UI but it has no features yet implemented to plugin except a couple of example stubs and some unfinished work I did at one time that was working to check external mail accounts.
Site Mail was planned to be a Site Office feature but it does not have to be, it could be implemented as a link from the user profile page, so it doesn't have to wait until the Site Office vision comes together.
Around the holidays at the turn of the year I also did some prototype work on the idea of re-doing Site Office with Silverlight and it showed some promise for the possibility of really rich UI but I'm torn a bit because the Silverlight plugin is not as widely installed as I would like. I have also done some substantial work in implementing the Site Office UI layout again using YUI instead of ExtJs but I have not completed everything to replace ExtJs yet.
For the moment my work on Site Office is on hold because I need to focus on things will increase revenue before I can have the freedom to work on that again.