30 days is more than enough to simply increase your existing subscription costs and remove free access
You may be a developer, but you clearly don’t know how the businesses side actually functions, especially if you’re self employed. You remove free access and you lose hundreds of thousands of users and millions of ad impressions, as well as plummeting user acquisition since people don’t buy apps. The subscription they had at the time couldnt (or barely could) cover API access. If you start charging more, you lose more users, and you also have to refund anyone that doesn’t like the increased price. You’d be lucky to keep any whole percentage of your userbase.
If it’s your sole source of income, your expenses are very likely to be largely inflexible. You’re telling me you could handle a 30 days notice to cut spending by nearly 100% while also scrambling to figure out how to completely change your monetization in a sustainable way? What if you have a mortgage? Car payments? Other loans?
It’s not as simple as flicking a switch. Even sole proprietors plan out their business 6-10 months or a year+ in advance. Giving them 30 days notice is telling them you don’t want them to exist in the first place under the guise of generosity.