20 years, not 10.

((((((((50*1.07+50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07 + 50)*1.07*0.04 = 30,000 per year income, after saving 50,000 per year and making 7% for 10 years at a 4% withdrawal rate, which is very aggressive for a retirement longer than the standard 30 years.

((((((((((((((((((50*1.06+50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06 + 50)*1.06*0.026 = 50,000 per year income, after saving 50,000 per year and making 6% for 20 years at a 2.6% withdrawal rate, which should be relatively safe even for a very long retirement.