As soon as the sprue turns frosty. That's what I do.
I dip and pour with a ladle into a two bullet Lee mold, and as soon as the sprue turns frosty, I knock it off and right back into the lead pot. Of course Lee molds are aluminum, and they don't over heat like cast iron or steel molds.
"I solemnly swear to tell the truth as I know it, the whole truth as I believe it to be, and nothing but what I think you need to know."
New Jersey Politician's oath of office