For enzymes go with what it says on the bottle since different brands contain different total amounts of enzymes. Usually its something like take one before most meals or two or three if the meal is really large or especially fatty or something like that. Go with the bottle's instructions, but you may need to experiment a little to find what works for you. When I started taking enzymes I experienced fewer cramps after meals almost immediately, but everyone's different!
Does your enzyme have lots of different kinds of enzymes in it? (It should be listed on the bottle - protease, lipase, cellulase, amylase etc.?) The more enzymes the more kinds of food (protein, fat, fiber, starches) it will help you digest, so the more valuable the product.
....and while I'm at it, here a good one on acidophilus (probiotic).
[Note: don't necessarily follow the dosages on these pages... go with the instructions on your bottles and your own slight experimentation to see what works for you! ]