We requested an assessment, because the DVLA stopped my husband upon being told (by us) he had a diagnosis, they weren't aware of how bad or not it was, because we weren't asked. His driving was still good.
When we spoke to the consultant she said much depends on your history and what you have done in the past to how quickly or slowly certain abilities deteriorate. It was a 7month wait for the assessment, they passed him with flying colours but it is down to DVLA Drs to approve and then it took 1month for him to receive his licence.