It depends on a whole lot of things. Is your app generating revenue. Is the developer a full time employee. Are the changes you want the developer to do simple

It depends on a whole lot of things. Is your app generating revenue. Is the developer a full time employee. Are the changes you want the developer to do simple cosmetic changes. Is the largest part of the value in the front-end or is there some complex orchestration in the backend? I.e: has the product never existed for lack of good front-end and the value is precisely in the UI?