What traction do you need to see from your product so you would consider quitting your big tech job?

I don't think that traction is necessarily the indicator that a person should be looking for to leave a comfy job. I left a good job for a startup with 0 customers, and no product built. If you're able to make it work for 6 months or a year and doing a startup will enable you to go through significant professional growth, then I think its a worthwhile investment of energy and time. Most startups fail, and thats ok, many startups don't get any real traction either. That being said, I think if you have a family or other dependents then starting a company should be a bit more considered, their opinions and needs matter and you leaving a stable job will effect them.