If no syrup is leaving the feeder and the bees are drawing wax then they must be flying and you have to have some kind of flow to make wax. If they have nectar coming in they won't likely take syrup and keeping it on will be a waste.
If they aren't flying and drawing wax then they are taking the syrup. Fermented isn't good and should be replaced. You can use EO's to help prevent fermentation, there are other things that can be used as well.
The amount of syrup being taken in depends of course on the hive size (in bees) and weather. In good weather a full ten frame can use about a quart a day. It is also possible that you have a moderate flow and the bees are using both. That's something you will have to find out?
"Success is not final, failure is not fatal: it is the courage to continue that counts." Winston Churchill