There are a number of YT videos where people have drilled holes on the lowest part of their muffler, giving the stagnant water a way out. Judging by how much water is coming out, I'd say it can be anywhere from a few hundred milliliters to a liter, maybe more... depending on the size of the muffler.

Notice that if a car in front of you is on an incline, you could see water dripping out the tailpipe. This could mean that his muffler is already likely filled with water and tipping it slightly allowed some of it to spill out.