10X doesn't ask for nor take 15% of gross income from their clients. That statement is not accurate. All of my projects through 10X have been considerably longer than 2 months.
I'll clarify: 15% of all contract programming revenue. As I'm a contract programmer, that's all of my revenue. But they did say they won't take a fee on your day job if you have one. They told me their average project is 1 - 2 months. That's from them, not me. Maybe you've been lucky. But if you're not giving them 15% of your contract development revenue then you may want to review your contract: you either got a special deal or you're in breach of contract. They even take their fee on any equity you're awarded.
I don't want to get into details of my contract with 10X. I'm not in breach, I certainly wouldn't be posting about it here if I was. I negotiated the terms of my contract with 10X when I signed up. All contracts are negotiable. I've worked through other consultancies and placement firms that have similar contractual terms. I've had full-time job agreements that included strict non-compete clauses and claims of ownership of any inventions or work I did while employed there, and sometimes extended one or two years after employment terminates, subject to applicable state law.