The amount you yield from each pump should be relatively equivalent to one of your baby's feeds. You may not wish to have oversupply as it may cause problems like mastitis.
It is recommended that you pump about 15 minutes on each side every 2-3 hours depending on your baby's needs. If your baby is bottle feeding, then the general rule of thumb is to pump every time your baby takes a bottle.
Fenugreek, like nursing milk are what we call milk boosters. Some boosters work for some mummies, but it really depends; you need to find what works for you. Like myself, milk boosters do not work for me but power pumping does and is very effective. You can read more about power pumping here: http://sg.theasianparent.com/power-pumping-an-alternative-way-to-increase-milk-supply/