You should aim to clean a bathmat at least once a week, and possibly more often depending on how regularly it is used, especially if multiple people make use of the bathtub or shower enclosure in a household.
Bathmats can prove a breeding ground for bacteria, as their positioning renders them unable to circulate air or dry off, which can lead to a build-up of mould and mildew.
For further information, take a look at our dedicated blog, The Importance Of Cleaning Bathmats.