He responds, "It is not for you to know the times or periods that the Father has set by his own authority."
[Aside: We have recent evidence that this pronouncement is still true. See http://bit.ly/iXbmtG]
Although he doesn't tell them the exact time, he does assure them that the Holy Spirit will give them the power they need to be his witnesses--and that their work will not be just for their neighborhood but to the ends of the earth.
They see him ascend. While they're looking up to where he has disappeared from them, two men in white robes ask them why they're looking toward heaven. They say that Jesus will return.
In his commentary on Acts for the Interpretation series, Will Willimon says:
The response of the disciples to the instruction, reproof, and the promise is exemplary. They gather to pray...They are told to be witnesses "to the end of the earth" and their first response is prayer. The action demanded of the church is more than busyness and strenuous human effort. Disciples have been told that the promised kingdom is a gift to be given in God's own time and that the promised Spirit is also by God's grace. Their mission requires more than even their earnest striving.