53.42 miles ; from the I-94/IL 394/I-80 junction in South Holland to I-94 in Northbrook (both Chicago suburbs). It's part of the Tri-State Tollway, so please have exact change.
I-294's original alignment extended into Indiana, overlapping with I-80 and I-90. In 1964, I-90 and I-94 were swapped in Gary, so that I-94 and I-80 shared the Borman Expressway; in the same year, I-294 was truncated to where I-80 and I-94 now diverge. 
In 1968, Indiana requested an extension of the I-294 designation. It would continue east along I-80/94 to IN 912, then go north on 912 and west on US 12 back into Illinois, eventually to reach I-94 again. This request was denied. 
Currently there is no direct access from I-294 to I-57. In August 1999, the Illinois tollway board's operations committee approved paying about $1 million to an engineering firm to design an interchange there. The mayor of nearby Riverdale applauds the plan. "It is such a natural connection that it should have been done when the network was first established. To have two major arteries that cross over each other and don't connect makes absolutely no sense." 
Thanks to Marc Fannin for clarifying some 1960s information.