When should a side business become a full-time one?

When it covers your living costs for three months running and the constraint on growth is your hours rather than your ideas. Before that, quitting buys pressure rather than progress.

In more detail

There is a second test worth applying: is there work you can clearly see would grow the business that you have no time for? If not, more time will be spent tinkering rather than building.

What to do about it

Test whether people will pay before building anything. A hundred people saying they like it is worth less than one paying.

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.

The mobile app angle

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.