About 40 km. (25 miles) but on the Sabbath (Acts 1: 12) it was fixed as not more than 2,000 cubits (about 1,040 metres or ¾ mile).
Citation for day's journey
Citation styles are based on the Chicago Manual of Style, 15th Ed., and the MLA Style Manual, 2nd Ed..
MLA
"day's journey." In A Dictionary of the Bible. Ed. W. R. F. Browning. Oxford Biblical Studies Online. May 22, 2013. <http://www.oxfordbiblicalcstudies.com/article/opr/t94/e2120>.
Chicago
"day's journey." In A Dictionary of the Bible. , edited by W. R. F. Browning. Oxford Biblical Studies Online, http://www.oxfordbiblicalcstudies.com/article/opr/t94/e2120 (accessed May 22, 2013).