When to add syrup and when to stop?

So I checked out both of my hives this weekend, I am in South MO and we have been having great weather, both hives made it through, I saw the queen, eggs and larva in each hive, also some nectar and bee bread. I removed the sugar candy boards, they barely touched them and will use the sugar for syrup. The temperature is above 50 now during the day and I am sure it will be fine.

My question is when to pull the syrup to make sure they only use it to build comb and not for honey?