As long as you like provided you pay the renewal fees and answer challenges about its use. A UK registration is good for ten years from the application date. After that it must be renewed – a simple online procedure.
The idea is that you should be using your trade mark on all the goods and services for which it is registered. After its been on the register for 5 years, you may be asked to prove that you are. This will happen if someone wants to use a similar mark and thinks your registration is a problem for them. Many registrations fall into disuse or were registered for far more goods than they have ever been used on.
If you have a US registraiton you will be required to use it before you register it and confirm it is still in use at regular intervals. This means the US register is a lot more informative than the register of European Union Trade Marks (EUTM). When you apply for a UK mark you must declare that you use or intend to use it. Not so at the European Union Trade Mark Office where applicants register their marks for great swathes of goods and services.