How to start / stop Dropbox at a specific time

Dropbox is not started at boot time, there is no init.d script to do it. The daemon is started at login for each user and the binaty is located in ~/.dropbox-dist/dropbox.

To start up the daemon you need to use the command

start-stop-daemon -b -o -c user -S -u user -x ~/.dropbox-dist/dropbox

and to stop it you need to use

start-stop-daemon -o -c user -K -u user -x ~/.dropbox-dist/dropbox

Where user is your username.

Source for this information is at http://ubuntuserverguide.com/2012/06/how-to-install-and-configure-dropbox-on-ubuntu-server-12-04.html. There is also a nice startup init.d script there, you might consider following that guide.