6 months: Regularly covering server expenses (which were ~$50-100/month at that point).
18 months: Would have paid for me to live nicely on the beach in Thailand.
30 months: Would cover my rent (and nothing else) at a nice apartment in a major city.
40 months: Passed the monthly take-home pay from my first job out of college.
That's for a SaaS subscription product that I built with the explicit goal of having a low-level income stream that I didn't have to put much time into. Early on, there were periods where I worked 80 hours or more per month to get the infrastructure ticking away to my satisfaction. These days, it requires maybe six hours of my attention each month.
So I guess the answer to your question is: longer than you think. But once you're there, it's pretty nice.