What percentage does the average investor make a year?

As in a stock investor.

The average depends on the time period you choose to look at.

If you had bought a Nasdaq representative fund in 2001. Then your average loss would be about 6.4% per year.

But if you had bought Nasdaq in 2003, then your average profit would be about 9% per year.

It's not just the average that matters but also the standard deviation. Standard deviation is what tells you both, the potential for profit and the risk of loss.

Depends on the year!
You can lose money for the entire year.

On an "average" year (no talk of recession, not really booming either) - I have no idea!

Cant be much more than 5% - 15%.

Long-term, the average value of typical stocks grows (inc.reinvested dividends) an average of 10-12%

One of your responders mentioned 10-12%. That has been the average during the last few years, but for the average investor over a long period of time a good return is closer to 8%. The average for the average investor is probably not that high. The average investor tends to make some not so good investments. The above average investor also for that matter. There are some mutual funds that have over a long period of time returned better than 12% annually. GAM 15.5% since 1980. ADX 12.4% since 1980. Just a couple that I know of. But they are the exception rather than the rule.

the average investor looses money.