Most insurance policies run for 6-12 months and can be canceled at any time. Each company handles cancellations differently so I would recommend asking about their cancellation process before starting a policy to avoid any fees. One thing to keep in mind is that you will need a US address to start a policy since rates are partially based on the location where you plan to keep the vehicle. It is also likely that you will not be able to activate the coverage until your vehicle is physically inside the US. You can shop for quotes online
or you can contact a local agent in the city where you will be staying.