How risky is it to keep most of my money in my employer's stock?

Very risky, because you're doubling down: your paycheck and your investments both depend on one company. If that company struggles, you could lose your job and see your savings crater at the same time, exactly what happened to Enron and Lehman employees. This is the opposite of diversification.

A common guideline is to keep any single stock, especially your employer's, under about 10% of your total portfolio. Concentrated employer stock often comes through ESPPs, RSUs, or 401(k) matches, and people hold it out of loyalty, tax hesitancy, or inertia. The fix is to diversify systematically: sell in planned chunks, use new contributions to buy broad funds instead, and be mindful of capital-gains taxes when trimming. The emotional attachment is understandable, but concentration in the company that also signs your paycheck is a bet you don't need to make.