Short term auto insurance is necessary for some people. For example someone could own a car here in the United States and live outside of the country. This person may need to acquire a short term auto insurance policy if they do not maintain auto insurance constantly.

The state of California DMV requires that vehicles must maintain at least liability insurance to be eligible for vehicle registration. Thus the idea of “short term auto insurance” in the state of California is not really a feasible option.

Major insurance companies standard policy lengths
Geico: no temporary insurance offered. 6 month policy standard.
AIS (insurance broker): shortest policy is 6 months
The General (car insurance broker): shortest policy is 6 months, typical auto policy is 12 months
Farmers: no short term auto insurance available
State Farm: no temporary car insurance

It seems like most major carriers, including insurance brokers that shop many different insurance providers for their customers, do not offer any auto insurance policies for less than 6 months in California. If short term auto insurance such as 1 month auto insurance is really required then it makes sense to check with each potential insurance provider individually to see what fees would be charged for early cancellation. The fee for early car insurance cancellation varies and it the car insurance cancellation rate itself can be variable depending on the length of time left on the policy at the time of cancellation.

Here is an example of how a 6 month auto policy with a cancellation might work:

The 6 month premium is 600 dollars. The policy is canceled after 1 month so the insurance provider charges a cancellation fee of 100 dollars. The remainder of the policy is refunded to the customer at a prorated rate. $600/6 = $100 per month, and 1 month of the policy has been consumed already. Therefore the insured would be entitled to a refund of $400 in this scenario.