Any headhunter with a sense of commitment to their client will place a period of whereby resignation will lead to a penalty.
This period will usually span between 1 to 2 months. Failing to adhere to this agreement will constitute as a violation of agreement and liable to 1 month’s wage of penalty to be paid to the agency.

It has 2 objectives.

1. Have the candidate commit to a decision made to accept the role offered by the agency’s client.

This is a very fair requirement for all parties. Candidates who decided to take up the role should have the same decency to stick to their decision and try things out until the expiration of the agreement unless health, safety and legalities are involved. For example, the selected candidate is required to break a law to fulfill his job responsibilities or take up safety officer roles without training or certifications when it is mandated by law.
Otherwise, it is very natural and basic for candidates to not resign from any roles they agree to take up. However, some companies require an absurd period like 3 to 4 months which also indicate alarm bells need to be raised. Why do they need to go beyond the norm of extending this period to 4 months? If this is the case, shun the company. The agency however should be left alone as they merely act out of their own interests and their obligations to their clients.

2. Ensure that manhours/efforts placed to secure the candidate to the role by the agency is protected against any violation by the candidate.

Agencies put in effort to run the head hunting, selection process and also the coordination between candidate and client should be fairly protected from any parties doing a renege on their promises. A candidate that joins a client company but resigns a day later just because they feel that the company is not their liking lacks the grit to hold out in times of adversity. An old fashioned virtue that some of the later generations consider obsolete, therefore bears no qualms making a mockery out of it. So think carefully if you want to accept the role. The interviews are good platforms for you to learn more about the job. You want to be treated fairly, so start by treating your agency and the company that is hiring fairly too. There is no era for good behavior.