Depending on the amount of tunnels wanted, two 10 pound bags of excavator clay may be suitable for a 40 gallon tank. it is recommended to set up your leopard gecko in a temporary tank until the excavator is completely dry. A heat lamp may be placed over the excavator to help it dry faster.
The dry time of the excavator clay depends on how much water was used and how thick a layer of excavator is placed. To speed up the dry time you can always put a fan or a heat lamp over the tank to help.