What are the risks pertaining to timing on long term index
The risks are countless for any investment strategy. If you invest in US stocks, and prices revert to the long term cyclically adjusted average, you will lose a lot of money. If you invest in cash, inflation may outpace interest rates and you will lose money. If you invest in gold, the price might go down and you will lose money. It's best to study history and make a reasonable decision (i.e. invest in stocks).
Here are long term returns by asset class, computed by Jeremy Siegel:
$1 invested in equities in 1801 equals $15.22 today if was not invested and $8.8 million if it was invested in stocks. This is the 'magic of compound interest' and cash / bonds have not been nearly as magical as stocks historically.
2) How large are these risks?
The following chart shows the largest drawdowns (decreases in the value of an asset) since 1970 (source):
Asset prices decrease in value frequently. Financial assets are volatile, but historically, they have increased over time, enabling investors to earn compounded returns (exponential growth of money is how to get rich). I personally view drawdowns as an excellent time to buy - it's like going on a shopping spree when everything in the store is discounted.
3) In case I feel not prepared to take these risks, how can I avoid
The optimal asset allocation depends on the ability to take risk and your tolerance for risk. You are young and have a long investment horizon, so if stocks go down, you will have plenty of time to wait for them to go back up (if you're smart, you'll buy more stocks when they go down because they're cheap), so your ability to bear risk is high. From your description, it seems like you have a low risk tolerance (despite a high ability to be exposed to risk). Here's the return of various asset classes and how the average investor has fared over the last 20 years (source):
Get educated (read Common Sense on Mutual Funds, A Random Walk Down Wall Street, etc.) and don't be average!
Investing in a globally diversified portfolio with a dollar cost averaging strategy is the best strategy for most investors. For investors that are unable to stay rational when markets are volatile (i.e. the investor uncontrollably sells their stocks when stocks decrease 20%), a more conservative asset allocation is recommended. Due to the nature of compounded interest, a conservative portfolio is likely to have a much lower future value.