Data has to be kept up to date and accurate under GDPR Article 5(1) so how frequently does it have to be cleaned?

As a Regulation the GDPR is law not best practice. Although it does not specify a frequency for data cleaning it does states explicitly in article Article 5 that “Every reasonable step should be taken to ensure that personal data which are inaccurate are rectified or deleted” . By ‘reasonable’ it is imperative to have a trusted data cleaning process in place to evidence that you are adhering to this requirement