No need
Just shut it off, no matter how full or empty it is. In fact, an empty pot takes a LOT longer to melt lead that's put in it. That's because an empty pot does not have full contact with ingots or scrap bullets as it heats up.
No need to test that last statement, but if you're really bored some time, record how long a ½ pot of lead takes to come up to temp. then next time you empty it to clean it, record how long it takes to melt loose lead up to casting temp.
Oh yes, it's sometimes time to thoroughly clean any lead furnace/pot. Dirt and oxides DO build up on the inside of the pot, trapped there by the weight of the lead. Draining it if it's a bottom pour into ingot molds, then tipping it when it stops flowing will get most of the lead out. Then heavy gloves to pick it up and tip it over, gets the rest of it.