Bullets that long and heavy, let is sit for about a count of four after the sprue hardens....longer than that won't hurt, but the sprue sets up hard and it's more difficult to knock the plate over.
HArd to be exact...if the sprue is ahrd to cut when the plate is knocked over, probably waited too long....if the mold collects a lot of lead smearing across the blocks, then proably opening too early.