I prune back my Santolina every spring in March. First picture shows it before pruning.
Without pruning it would get leggy and ugly. Pruning makes it more dense. Below after pruning:
Doesn't look very handsome NOW, but believe me it will look great - round and dense - in 2 months!
I am not an expert - just share my experience with you and I would be very happy if you share your experience with me, by adding comments.
I have read somewhere, that it can be pruned in the Summer, after flowering, to keep it shaped nicely and flowering every year.
I did not like yellow flowers, so I was not particularly looking for flowers of Santolina chamaecyparissus.
Assuming that I would like to promote flowers of Santolina - what shall I do?