You can agree an agency relationship either for a fixed term or indefinitely.
If the agreement is for a fixed term, then it will normally be terminated at the end of that period. However, if you allow the agent to continue to act for you after the term expires, you will have effectively entered into an indefinite arrangement.
For an indefinite arrangement, the agreement should state what notice period there will be if either you or the agent wish to terminate the relationship. In practice, if the agent wishes to terminate the relationship, you may wish them to immediately cease acting for you. You may, however, be obliged to pay the agent some compensation to cover this.
For self-employed agents, under UK and European law you must give a minimum notice period to terminate the relationship: one month in the first year; two months in the second year; and three months thereafter. The notice period can be longer if agreed.
You can terminate an agency relationship without notice if the agent breaches the contract with you - for example, if the agent fails to provide you with the information you need to fulfil sales.