Don't leave it until the rating has expired, even by one minute, you need to do a test to get it back (expensive).
As the odd one points out you get three months to get the papers signed so it should not be a problem. The one that really gets me is you can do the flying 11 months 28 days in advance of it expiring then you must twiddle your thumbs for 9 months until the examiner can sign your book -and if you forget and leave it 12 months then you need to take a test. To really mess something up you need the CAA.