My husband was an early employee at a company that was acquired in a mid-nine-figure deal. The founders walked away with at least a hundred million each. Employee #1 got a low seven-figure payout, after about ten years of grueling overtime. Keep in mind this was actually a company that is IMO unusually fair and considerate toward employees.
You really need billions of dollars coming into a company before non-founders have a chance to make fuck-you money.
Even if the start-up is a huge success, the chances of you making substantial amounts of money are low, unless you have founder-level equity.