It's a really good question without much of an answer. In a sense all of the present-day IE languages are modern PIE. Languages don't change at the same rate. Did PIE split into two daughter languages or more? Or did a whole bunch split off. There's no written record of PIE, and it's very difficult to associate archeological artifacts with languages. (E.g., there's some who doubt that the Celts occupied the wide area across Europe that they're usually thought to have. NB: I'm not saying this theory is correct/incorrect. I don't think there's enough evidence either way.) Take Latin, when it stopped be the official language in the western Roman empire, did it "split" into what became Italian, French, etc.? It remained in place and gradually started to differentiate into its daughter languages? Latin survived, but only as a kind of artifical language for academic / ecclesiastical discourse. A lot of ink (and ultimately blood) was spilled over which "race" speaking a daughter language represented the PIEs (Aryan) or which daughter language was closer to the ancestral to the mother tongue (PIE). Lithuanians today claim that there's is mainly due to some phonological, grammatical, and lexical claims made in the 19th century. Most IEists today don't buy it. There was a Flemish scholar named I. Goropius Becanus who figured out that the Antwerp dialect of Flemish (his language coincidentally) was the language of paradise. Chauvinism of this sort is still with us today. IEists aren't even sure where PIE originated, though many today have reached some agreement on the steppes of Central Asia.
There's been some new work done at UPenn by Donald Ringe and some colleagues: it's based on algorithms from biotech called cladistics. But he just gives relative dates of splits between individual branches.
http://www.ling.upenn.edu/~dringe/home.html
