Yeah I drink a consistent amount of Monster every day, and I work out very regularly. I have subscriptions for Monster and for my particular brand of whey. I can't think of anything else -- though cat litter is a perfect example, too -- that I'd sign up for a subscription for. Even my deodorant seems to be used at different rates.
But that's a knock on me, not the subscription service. If I kept better track and knew for sure that I use 1/60th of a 12 ounce bottle of shampoo everyday, or whatever, then I'd be more inclined to subscribe to that once every two months.