If you don't leave some lead in the pot, it takes a lot longer to melt lead for your next casting session. Reason? Contact with the heated pot is what melts the lead. If you put ingots into the empty pot, they are not contacting the inside of the pot in very many places. An inch of lead in the bottom will melt faster but if the pot is left ½ full, it'll melt even faster,,(because the heating element is positioned just below half way down).
I empty mine about once a year, or after ten casting sessions. I have 2 of them so it's not all that often. There's a good collection of crud really nailed onto the sides and bottom of the pot. I take it outside in a good wind so I don't breath those dangerous lead oxides. I take the valve and lever off to be able to get at everything. I use a big wire wheel and wire cup brush in a high speed ¼ inch drill motor.