How long should you spend on the initial version of the app? In my opinion, 4 months is the right amount of time. This considering that you are building an iOS / Android app on a cross platform tool like React Native for which the API also needs to developed.
Anything less than 4 months and you start to cut corner and rush features. 4 months should give you enough time to build the list of features and ensure that it works on both platforms.
Any longer than 4 months and you start to feel like you have been working on the project for too long without getting any feedback from the user. It’s important to get feedback as soon as possible.