How long does it take to sail from California to china in a 30ft sailboat during summer time conditions?

As crzyfran has said, it is a long trip and unless you have some open water sailing experience it would not be the first trip i would pick. As for a 30 ft sailboat, some of these boats are much better blue water boats than others. I sail in the Gulf of Mexico on a Pearson 30 ft boat and this boat is a decent blue water boat at 10,000 lbs. Sails, keel, electronics, crew all these things and more come into play on a voyage like what you ask about. The time of year you plan to leave and the route you plan to take also play into you voyage. Time in port at the places you plan to stop at and time in port for any broken parts you may need to replace could be a factor in this trip. I have sailed in the Gulf in much smaller and lighter boats but unless you are prepared for the unexpected I would be very careful. I have never sailed in the Pacific so what I say is based on the Gulf of Mexico. I have 25 years or so sailing and with all the right conditions I would find a trip like what you describe interesting, with ALL the right conditions.