Inception and expiry dates of the contract are stated in your policy.
Some insurance providers offer short contract periods (1 year), others automatically offer longer periods (often 5 years).
You can find out how and when it is possible to switch insurers before the contract expires here.
Avoid long contract periods in order to stay flexible and profit from more favourable offers. This way, you will save more money than the small discount you receive for long contract periods. You can insist on the right to terminate annually with companies that do not offer one-year contracts. This has to be stated in the policy.