The UK Visa usually has a validity period of 6months. Meaning the holder is permitted to stay for that amount of time. In some instances, the holder could be allowed to stay for up to 11 months if their visit involves private medical treatment. For academics coming for sabbaticals or research, your spouse or children are allowed to stay for up to 12 months. 

You MUST leave the UK on expiry of your visitor visa as failure to do so could have negative impressions on your immigration record.