After about how many years of formation of earth, life appeared on this planet?

Life took approximately $500$ million ($5 x {10}^8$) years to appear after the formation of the earth. Thus life is thought to be evolved almost $4$ billion ($4\mathrm{x}{10}^9$) years back from now, as the earth was formed $4.5$ billion ($4.5\mathrm{x}{10}^9$) years ago.