Right to work checks should be carried out before the worker starts carrying out work for the employer. For someone who is an EU, EEA or Swiss citizen who provides their passport or ID card as proof of their right to work, then provided the initial right to work check has been carried out correctly, it should not need repeating.
More guidance on carrying out right to work checks correctly is available here.
NB: There may be exceptional circumstances in which an employer knowingly employs individuals who have not obtained the necessary leave. This could leave the employer open to both civil penalties and prosecution, regardless of whether right to work checks have been carried out.