There are no particular number of days you must work for GOC registration but your professional competence could be questioned if you were say only working a few days a year. They have reduced fee for those earning below £12k a year. See http://www.optical.org.uk
However there is a minimum GOS activity to remain on NHS England Performer List to do GOS work and to be able to claim your annual CET grant. It isn’t very high but they can and should remove you from the list if you have done no work your area teams region in previous 12 months. If that is likely to be the case you should contact them and they will be sympathetic to allowing you to be retained if you have good reason and intend returning again, eg maternity leave. But if you are not practicing for a long time they will ask you to do a return to work course to refresh and update your skills and knowledge.