A few years ago I had the same question and found some answers looking at definitions of "reading" in academic articles (I wish I could remember the specifics).

I remember one article using something like...'reading a 4-5 simple word sentence (assumed non-sight words, no pictures)'. That seemed reasonable to me. At the time I didn't really know what my DS2 was capable of so I typed short, random sentences on my computer (e.g., "The bridge fell on my foot") to get a sense of where he was at.