1) Need the money that comes from a day job.
2) No longer like the work you are doing, and can afford to stop doing it.
3) Can think of something better to do.
For me I was working very hard on something I enjoyed but nobody else cared about. I needed money, so I went and got a day job. If I didn't need money, I would have kept doing my project.
I'm now buried so deep in this domain it feels like it's supposed to be a long term career. I enjoy my work and it's the best and most valuable (IMO) work I've ever produced.
We don't _need_ the money as we live a very simple life, but not being able to earn a living is having an effect on my self-esteem.
If it works, great! If it doesn't work, the consequence will be based on how deeply you care about this project and if you can just transfer your skills to something new or not.
Wishing you the best :)