TSgt at 16 years: At what point does a finance “side project” become a business I need to report?

I’m looking for insight from those who have successfully balanced a side project with active-duty life.

As I enter the last quarter of my 20 years and sit just months from finishing my Finance degree, I’ve been looking for ways to help family, friends, and co-workers with some of the tools I’ve gathered along the way. I decided to combine three of my passions—finance, computers, and personal privacy—to create a tool that helps people track their net worth without continuous subscriptions or data-mining.

I spent the last several months coding the program, and when it came time to share it, I decided the only responsible way to do it was through the official Microsoft Store certification process. By using the Store, I’m leveraging Microsoft’s distribution and security infrastructure. They handle the code signing and safety scans, which lets people try the tool without worrying about 'unsigned' files, while I keep all user data strictly on the local machine. I figured this was the best way to offer promo codes so people could try it safely without their antivirus flagging a random file.

Once the program was live, I started to think: at what point does a hobby become a job?

Within the first few weeks, the app has sold exactly once… for 5 dollars lol. But hypothetically, if it takes off, do I need to proactively file an AF Form 3902 even if it’s just passive royalty income? Also, for those with similar experiences, at what earnings threshold did you decide to file as an LLC or treat it as a formal "second job" vs. just a hobby that pays for itself?