First:
Use this DM Plugin:
http://www.bailopan.net/cssdm/index.php?page=download
Second:
When you activate the mapchooser.smx file from Sourcmod you can set when the mapchooser starts:
Fraglimit, Timeleft or Winlimit...
You can set all three...
So a vote starts 3 min before the maps end, or 5 frags before the fraglimit is reached.
I hope this can help you...