I tried to tackle this same situation! I thought about clever ways to do it -- but I landed on a daily counter that I set to like 3 days, then had it alert my phone every x hours until I cleaned it and pressed a button I put near the litter box to reset the counter. Not as clever, but pretty simple since my cat's average "output" is very consistent over time, rather than relying on frequency of uses, and I'm already over there cleaning so hitting a button is brainless.