Most photocard driving licences are valid for 10 years from date of issue, except for the over 70's who need to renew every 3 years. However, some medical conditions may worsen over time so the DVLA overcomes this by issuing time restricted driving licences. Licences could be issued for 1, 2, 3 or 5 years. Before issuing the new licence, the DVLA will ask for updates on your condition from yourself and relevant medical professionals. They will then review the information and decide if a licence renewal can be authorised and for how long. In some situations, the DVLA may request that you have a driving assessment before a licence is issued, this is so they can find out whether your condition is affecting your driving. There are many conditions that need to be reported to the DVLA but not all of them will result in a short-term licence.
When you first apply for your provisional licence, you may have a medical condition that needs to be declared to the DVLA and results in a short-term licence being issued. This needs to be factored in when preparing for your theory and practical tests as you cannot take a test without a valid licence. For 1 and 2 year term licences this can cause significant disruptions to your driving and extends the time needed to get through the tests.
It usually takes several weeks, but can take several months, for the DVLA to complete the necessary checks and issue you with a licence. It's important to keep a check on the licence expiry date, although you should receive a reminder about your licence renewal, it may get lost in the post and it's ultimately your responsibility to ensure that you hold a valid licence. This is one of the many reasons why you need to keep your address details up to date on your licence, it's very easy to do online.
Find out more here:
https://www.gov.uk/driving-medical-conditions