Can I change how often the droppings appear?

Yes. By default, Mouse Droppings drops 1 to 4 marks every 3 seconds. You can change the interval using a command-line parameter. For example, DROPPINGS.EXE 10 sets the delay to 10 seconds between droppings for a slower buildup, while DROPPINGS.EXE 1 cranks it up to maximum mess.