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100% of my income comes from freelancing. I build web apps and APIs in Node.js. I charge an hourly rate and I bill once every 14 days. I only take on projects that are going to be at least two months in duration and I give my clients 14 days to pay after being invoiced.

I don't usually have much spare time, although it's not because of my billing practices.

If you're doing projects that are only going to last a week or two, just get 50% up front and 50% on completion.