I think what you're asking is how do you know when you've reached true product market fit. There is a ton of literature out there on this and if you search for "product market fit" you'll probably have more than enough information to apply to your own business, but it depends a lot on the product and the market you are in. Some businesses, like those going after B2B, small businesses, have a harder time reaching "critical mass" because those markets are extremely fragmented. Others like Facebook, that targeted small, close knit communities with high k factors and word of mouth virality (colleges), can gain traction over night and can be very apparent. I would look for inflection points in key metrics that you are watching for your business: CPA (cost per acquisition), Organic/Non-paid acquisition, referrals, key retention metrics etc. to see if your k-factor is increasing. This can be a good leading indicator that you have reached product market fit.
Good luck and if you'd like to chat more let me know!