As you can see above, returns for the index varied significantly each decade. Look at your right to see more in depth analysis of the returns.
S&P 500 index has been published since 1957, which is the weighted index of the 500 large-cap common stocks. Although the official data is only available from year 1957, we can still calculate the Historical return for s&P 500 index from earlier years by taking the very same companies. This exact thing has been done on the below graph which is the calculation starting from year 1926.
The average return since 1950 was 10.8%, but if the inflation is subtracted then the real total return is 6.8%. However, this by any means does not guarantee that this is the percentage you will earn. Calculating the expected return will give you a better estimate, but even then it does guarantee anything. To show you an illustration, if we were to compute the expected return over the past three year period with 10% return for each year, then the average return would be 10%. (10%+10%+10%)/3 = 10%.
But if we were to calculate the expected return for next year and if we knew this information:
Probability of a Boom = 0.2, Return if occurs = 15%
Probability of a Recession = 0.2, Return if Occurs = 0%
Probability of an average stage = 0.6, Return if occurs = 10%
The the expected S&P 500 index return for next year would be, (0.2*15)+(0.2*0)+(0.6*10) = 8%