Move up to 5 lbs when 3 is no longer challenging, even just for particular sections. (I found the flies easy, so went up just for that bit for a while). At the same time that you're experimenting with weights, move up a level - you can even swap days/weights.
I can't remember when particular results happened... I know within the first week, I felt stronger. By the second week, I felt tighter and had lost a few pounds. Diet is (as ever) important in seeing results.
Goal - 135 lbs
April 2011 - 145 lbs
August 2010 - 178 lbs
C25k, DVDs, swimming