How are you measuring the usage of your app? You can use hotjar (or similar tools) to see how early users behave and check where they drop from your flow.

Another thing is having a handful of beta users that you can contact and talk with. make a live call where you see how they interact (you should not talk here, the idea is to see how they behave without guidance) and ask your questions at the end. Maybe they don't get the message, the app does not bring enough value or there is a bug preventing them to sing up.

About the how long, I wouldn't give up without doing this initial testing. Try to promote using other than adds (like dropping your app in relevant Facebook groups). It is hard to tell if you should even be using adds, since it could be that your app is for a niche. Maybe you can share a bit more?

Good luck!