There's no guarantee at all. In a really bad year (2008?) you might get -50% or worse. Then you might get some really nice returns in the years after (or not!). If you average over several decades, historically, the returns have been around 7 or 8 per cent per year, but the standard deviation is enormous. Just look at a long-term chart of e.g. the S&P 500 at https://finance.yahoo.com/chart/%5EGSPC - click "Max" and "Settings" -> "Logarithmic" (you'll want a logarithmic axis so that equal percent changes are equal distance on the plot). You'll see that on average it went up over the decades, but between June '07 and February '09, it lost over 50%, and tripled since then.
I encourage you to read up on this, but someone else with more knowledge should recommend some books.
All of this is correct, but the standard S&P 500 index doesn't include dividends, so your typical index fund will (should) do 1-4% better each year than the S&P 500. The S&P 500 does have a lesser-known version that includes the total returns: https://www.google.com/finance?q=INDEXSP%3ASP500TR&ei=WWJNWK...
Banks operate on a completely different model so it's not a useful comparison, but the answer is that they are borrowing at ~0% and lending 2%+ so they view the world differently
banks are heavily regulated and cannot just put deposits (the money you put into a bank constitute a loan to the bank) into stocks. They have to invest more conservatively, e.g give money to solid companies or hold low risk bonds. See Basel-rules to learn more about risk weighted assets (RWA): https://en.wikipedia.org/wiki/Basel_III
You should get inflation + some economic growth + risk premium for holding equity.