Grocery store decorators: how long do you guys refrigerate your premade buttercream before piping flowers?

I need help with this. I have a cake order that wants roses on it and I can’t figure out how long I should refrigerate the frosting to firm it up. I tried to do it for 30 minutes but then when I went to pipe the frosting, nothing came out. How long should I keep it in the refrigerator to firm it up enough to pipe the roses?