How do you know when to stop and try something else?

When you have tested the three things you believed would fix it and none of them moved the number. Stopping then is judgement; stopping before is impatience, and stopping much later is sunk cost.

Why the answer is what it is

Write those three tests down before running them, with what result would count as working. Deciding afterwards whether a result was encouraging is how businesses stay alive for years without ever being alive.

What this looks like in practice

An idea is worth almost nothing on its own. What is scarce is somebody who will keep going after the first version does not work, which is every first version.

The best ideas usually come from a job somebody already had. You notice a problem because you lived inside it, not because you brainstormed.

A market that already spends money is easier than a market you have to convince exists. Competition is evidence, not a warning.

Where a mobile app comes into it

If the idea is a shop, the sequence matters: sell first, prove people come back, then build the app. An app before the customers is a cost with nothing to do.