It would depend on the usage of skills, whether or not you sit down to fight, and how long you burst.

Sitting down at a spot like wind bees or frogs generally saves you alot of syrup when bursting. Also using soul dodge on a character saves alot of syrups. Generally when I am setting up my burst team, I have my characters (except my main killer) riding comfortable saddles and usually set my water as the party leader and make her sit down. I make my killer use soul dodge and since he's quite high and have some int (from potential) he completely sucks the sp from the mobs in the first 2 rounds. If I do this and sit down and use a skill that doesn't require much sp like hagendis hit, I can go about a week with only 5 syrups. However, if I am not doing this, and I intend to go for a long time (say 8 hours or so) and use fire ds and soul dodge, I usually have about 6 stacks on him, just incase. For my other characters, I usually have around 5 or 6. I would recommend you to have as many stacks of syrups on your characters as you can spare. It's always better to be over prepared rather than being under prepared.