The contract period is usually for 12 months with the fees being paid in advance. Exceptions are top-level domains such as but not limited to .ag and .co.uk where the minimum registration term is 24 months.
Cancellations are possible at any time. However refunds, of the yearly fee, are not provided by the domain name providers. The subscription is automatically renewed for another year if notice of cancellation has not been made at least 6 weeks prior to the expiration date.