To be safe stay within 50 nm of either side of the icon for 48 hours. Then head for home once the icon turns grey. Do not radio in a status report or you will get a second objective (ordinarilly you would want that, but with no torps left it's best to get home)
You will find these threads very handy over the next little while, as the manual leaves a lot to be desired.
http://www.subsim.com/radioroom/showthread.php?t=111509
http://www.subsim.com/radioroom/showthread.php?t=108931
JCC