This is all just speculation based on the Wikipedia entry for LEGO and some of the sources linked within it.
In 2008, TIME reported that the LEGO Group estimated that they had produced about 400 billion LEGO elements.
In 2006, Bloomberg reported that LEGO was producing about 15 billion elements per year. ()
In the Wikipedia entry, it is stated that "Annual production of Lego bricks averages approximately 36 billion," but that statement does not have a citation, so I don't know where it came from or when it was referring to. However, I would guess that it is a more recent estimate than 2008.
If we use the 2006 estimate of 15 billion and assume 2019 for the 36 billion estimate, we can work out an annual production increase of about 1.6 billion. Working from that, we can get a number around 706 billion total elements produced. And it looks like they'll hit 1 trillion elements sometime around 2026.
Again, this involves a lot of speculation and assumption, so don't quote me. ;)