How long do you get to use a UK standard visitor visa from time of issue or is it valid for entry indefinite

The UK visitor visa is normally valid for 6 months from the date of issue (on rare occasions a visitor visa may be issued for a shorter period). You can enter on any date during that period and must exit on or before the expiry date.
You cannot apply more than three months before the intended date of travel.

https://www.gov.uk/standard-visitor-visa