The logic is simple - count your actual spending. If a subscription costs, say, 200 rubles, and you're saving 20%, then you need to spend at least 1000 rubles a month just to break even. After that you'll start actually saving. Honestly, most people don't pull it off - you need stable orders.
I'd suggest tracking for a month or two without a subscription: how much you really spend on delivery, how often you order. Then you can do the math on paper. Also keep in mind - different services offer different conditions. Some give cashback only on food, some on everything, some require a minimum order amount. All that affects whether it's worth it.
Personally, I wouldn't get a subscription if I order irregularly. In 2026 there's plenty of these services, you can switch between them or catch periodic deals without being locked into a subscription. If you want to save money, just pick places with cheaper delivery or go to the store yourself - sometimes that's faster than waiting for a courier.