I usually treat my cages and racks once every 3-4 months regardless whether I see a mite or not. As prevention I pre-treat my Aspen at least 24hrs before using. Thankfully I haven't had a mite problem in years.

You can treat enough Aspen to cover 2-3 tub changes and treat at a once. Place the Aspen in a plastic bag, spray for a few seconds, close the bag and shake. Leave bag closed for 2-3hrs, open bag and place Aspen in a Rubbermaid to air out till the next day. I keep the rubbermaid covered after it's aired out 24hrs with it's lid and as needed I pull from the bin to place in clean tubs.

Or you can have an extra tub with clean substrate already pre-treated set for use. When the snake makes a mess in it's current tub just move to the clean/treated tub and add clean water bowl and accessories.

For my cages or tubs that paper substrate, I have pre-cut sheets treated and ready for changes.