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I looked at the competitors, the fact that there were many told me there's other people making money.

I tried their products out and decided I could do better myself.

I gave myself a fixed amount of time to get an MVP out in the wild.

In my opinion, you'll never "justify" the time. The only thing you can do is make your best guess, look at the evidence and just give it a shot.

If you're worried about time, sand box it. If you can't make it work, re-evaluate the project (move on or keep trying).