Depends on all sorts of factors in your own life:
- Do you have kids?
- Do you have a mortgage?
- Do you have a career or a this'll-pay-the-bills job?
- Could you walk into another job if you needed to?
In my own case, I quit after six months as an affiliate when:
- I had money in the bank for three months comfortable living
- I was earning more from affiliate work than my day job
- I had a substantial amount of commission earned (and due to be paid) from super-reliable merchants/networks.
As soon as I quit my day job, my affiliate projects started to really take off.
Your mileage may vary. In fact, I'd suggest that my benchmarks were a little free and easy with hindsight, but then in my case I was in a difficult position with work and had to leave anyway, so...
Whatever your circumstances, I think the key thing is to have a backup plan worked out (with financial detail) lest the might Google hammer comes down on your projects.