Usually, your UK Standard Visitor visa is valid for six months only, but you may be permitted to stay longer in the following cases:
If you have come to the UK to receive medical treatment.
If you are an academic, your UK Visitor visa can be extended for up to 12 months in total, the same applying to your family members.
If you are a medical graduate doing a clinical internship or re-sitting a Professional and Linguistic Assessment Board (PLAB) test.
Besides, if you plan to have several short-term trips to the UK in the coming years, you should consider applying for a long-term UK Standard Visitor visa for two, five or even ten years. Note that if you visit the UK regularly and stay for long periods (longer than 6 months a year), the Home Office may consider this an attempt to settle in the country and will cancel your visa.