In situations like this, the appropriate course of action depends on the specific context and the relationship you have with the person you are staying with. Here are some general guidelines:
Communicate openly: If you feel comfortable with the person and have a close relationship, it's usually best to communicate openly about the issue. Politely bring up the topic and ask if they would mind if you cleaned the bath/shower. They will likely appreciate your willingness to help and maintain cleanliness.
Respect their space: If you have a more formal or distant relationship with the person and don't feel comfortable discussing such matters, it's best to respect their space. You could try to avoid using the bath/shower until it's cleaned or find alternative arrangements for your bathing needs.
Offer to help with chores: Regardless of whether you address the bath/shower directly, you can show your appreciation for their hospitality by offering to help with other household chores. This gesture can demonstrate your consideration and willingness to contribute during your stay.
Maintain personal hygiene: In any case, make sure to maintain your personal hygiene. If you find the bath/shower not suitable for use and can't clean it, consider using other facilities like a nearby gym or a public swimming pool until the matter is resolved.
Remember, it's essential to be respectful and considerate of the other person's feelings and preferences. If you're unsure about what to do, observe their habits and follow their lead in terms of cleanliness and household chores. If you're staying for an extended period, the situation might naturally resolve itself as the person might clean the bath/shower regularly.