The markets are currently some 20%+ lower than they were at the start of the year. Those invested are nursing considerable losses which demonstrates the benefit of being patient this past few months as we waited, cash at the ready, for something like this to happen.
The question now is focused on trying to measure the extent of this downturn, and this is not easy. One method that may give you some assistance is the so-called "Buffett Indicator" which is the Total Market Index Value/GDP. When applied to the US this stands today at about 119% which means that despite the large falls to date the market is still overvalued. If you believe that where the US markets go the world tends to follow then this indicator may be helpful. In 2009 this number fell to as low as 51% so by that yardstick this current fall is not yet over.
We should keep in mind that within this there are many good companies shares currently representing excellent value so opportunity abounds.