Short answer: 5 months with my new startup (to reach about $1500/mo revenue).
Longer answer: I've completely immersed myself in startups for around 3 years and spent a full 1.5 years on a startup which never reached ramen profitability nor got funding. I don't think I'd have been able to reach ramen profitability in 5 months without the learning I did in the previous one.
I also write weekly on my personal blog and almost every post has some form of reflection comparing the previous startup with what I'm doing for my current startup: http://joel.is
Longer answer: I've completely immersed myself in startups for around 3 years and spent a full 1.5 years on a startup which never reached ramen profitability nor got funding. I don't think I'd have been able to reach ramen profitability in 5 months without the learning I did in the previous one.