After an installation has been completed, it should be registered with FENSA within 21 days.
You should only register an installation with FENSA when you consider the work to be complete and compliant to all relevant Building Regulations. If any snagging or other works like rendering are outstanding, you should refrain from registering the job with FENSA until this has been completed.
Remember when you register an installation with FENSA you are making a legal declaration it is compliant to all relevant Building Regulations.