If history repeats itself, as it has for the last four Bond films, then we will get a press release by mid to late summer detailing the director and, at least, some of the actors attached. In the fall there will be a press conference revealing the title, a basic log line, filming locations, etc. Production could then start as soon as a few of hours later or begin a couple of weeks later.
I'm really hoping for a 2014 release. The news that Mendes is not returning is actually a good sign that production will start sometime this fall/winter. Most spoilers or casting news is probably days, if not weeks, old.