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!).

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 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.