Section 12502 of the California Vehicle Code, subdivision (a)(1), states that you are not required to have a California license to drive if you are "A nonresident over the age of 18 years having in his or her immediate possession a valid driver's license issued by a foreign jurisdiction of which he or she is a resident . . . " We could not find in this or any other section of the CA VC a limitation to how many days you can use your foreign license in California, so long as the foreign license is valid and you do not become a California resident.
If you make your home in CA or take a job and you want to drive in California, you must apply for a California driver license within 10 days. Residency is established by: voting in California elections; paying resident tuition at a California college or university; filing for a home owner's property tax exemption; obtaining a license; or receiving any other privilege or benefit not ordinarily extended to nonresidents.
So if you stay in California for a job, or become a CA resident according to any of the other definitions residency, than you would need to exchange your foreign license for a California driver's license.
The CA Department of Motor Vehicles notes that visitors over 18 years old with a valid driver license from their home state or country may drive in California without getting a driver license as long as their home state license remains valid. They do not mention any certain time period you are limited to driving in the state with your international or foreign license as long as it is valid however you may want to contact the CA DMV directly to see they can give you specific information on how long the state allows a person to drive in California with a European license.
Keep in mind that the State of California does not recognize an International Driving Permit (IDP) as a valid driver license. California only recognizes a valid driver license that is issued by a foreign jurisdiction (country, state, territory) of which the license holder is a resident for international visitors.
The United States, Canada and over 100 other countries are party to the International Convention on Road Traffic. This Convention is intended to establish uniform reciprocal privileges on matters such as drivers' permits. The agreement also requires the US and its states to allow a foreign driver admitted to the US to drive a private motor vehicle for one year on certain conditions. So you may need to see if California keeps to this 1 year limit or if they have their own guidelines or rules for how longer foreigners to the US can drive with their foreign license.