Google CA penal code section 1203.4 (expungement statute). Typically, you have to have successfully completed probation (in your case, June 1, 2013). However, depending on the jurisdiction, often times a motion can be filed to terminate probation, based upon need, at the halfway point (18 months).
At least one year must pass before you can expunge a case. In addition, you can not expunge a case until your probation expires or is terminated early. To have a realistic chance to get your case terminated early and expunged on or after June 2, 2011 you should hire an attorney.
There is no legal requirement that you wait a year, or 18 months, to apply for a dismissal under Penal Code §1203.4, as the answers above indicate. The judge could terminate your probation tomorrow and grant the dismissal.
As a practical matter, judges don't usually terminate probation early "just because," but if you have a good reason -- such as employment or educational opportunities -- they can sometimes be convinced to do it.
You should also be aware that a so-called "expungement" doesn't really expunge anything.
Under Penal Code §1203.4, a person who is granted probation can apply to have the case dismissed after probation is complete. This is sometimes called an expungement, but the conviction is still a part of the court's public records, which will also show the case was dismissed. You still must disclose the conviction when applying to be a police officer or for certain other jobs, or if asked on an application for a state license or to run for public office. Your prior theft conviction could be used to increase the punishment for any future theft offense.
BEWARE of law firms that promise to "clear your record" and charge thousands of dollars. The process of filing a Penal Code §1203.4 petition is pretty simple, and you can do it yourself with forms available from the court clerk's office, especially if you completed probation without any problems. If you had a probation violation along the way, you may want an attorney's help, but the guys who charge thousands to file simple paperwork will probably overcharge you for that, too.
California Labor Code §432.7 says employers can't ask about any arrest that didn't result in a conviction, inquire about it from other sources or use it in a hiring decision.
Some attorneys interpret this Labor Code section to mean you don't have to disclose a conviction that was dismissed under Penal Code §1203.4, but I usually advise clients to disclose it, with an explanation that the conviction was subsequently dismissed.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.