It's on the back side of the engine between cylinders three and four.

It's rated at 100,000 miles.

 

5 years/100,000 miles... It was due for a change 2 years ago.

I know this is an old post but just in case someone comes across it and still looking for location of the engine block drain for coolant. Here is the video I made (2004 corolla LE). Hope it will be helpful to some.