Being a new plant with you I would suggest bringing it in when the nights are going to average 45F the first year. Following years if still that healthy leave it out a few nights at 35F. Don't be surprised if it pouts a little, some dry leaf, etc... It is normal and if so water it slightly less for a couple weeks and forget the fertilizer. It will come back with a lot of new foliage and then resume normal care.
As John
@Mellow Mullet said, and trust me he is PRO with Serissa that is most likely a year old. The larger one I am working in this post is 3 1/2 years old with a 1 1/2 inch trunk at the trunk line above the roots I exposed. Great start you have there, and as me and him discussed in your thread lighter work is in order this time of year for us more Northern Folk. He grows all year down there - Wonderful plants in every aspect but his weather allows it.
Grimmy