Fenugreek works, to varying degrees.
The most recommended dosage is 2 caps three times a day. However, really, the right dose is when you get a supply. For me, it took 7 caps three times a day to really get a good supply (but I am weird like that, I would not recommend unless under the supervision of a lactation consultant). According to Dr. Jack Newman, you keep going until you can smell the maple syrup-like smell in your secretions and skin. I did not get this until I took fenugreek myself...you see, it makes your sweat smell a little (but only to you, it is not an offensive odor). Most people say it is like maple syrup but to me it smelled like sweaty socks (slightly!). I think the best way to describe it is, whatever you smell once you open the container of fenugreek capsules, that is what you should be smelling on you once you have the right dose. I would just take a whiff of my underarm or my forearm to make sure my dose was right.
If it works well for you (and keep it mind it does not work for everybody) -- you should feel a healthy boost in supply in 1-3 days.
Fenugreek should not be used if you have asthma or tendency toward hypoglycemia. These are the most important issues to remember but there are other contraindications that I don't recall right now. I would research this before starting it.
In addition to fenugreek you may also consider adding other herbs. For example, the fenugreek did not work so great when I used it alone but when I added 3 caps of blessed thistle three times a day I was leaking milk. In addition, other herbs, such as fennel, assist with letdown -- that worked wonders for me. Many lactation consultants sweat by the herbal combos, the most popular being fenugreek and blessed thistle taken together.
In most people fenugreek either works wonderfully or not as great. It is tolerated by most with relatively few harmful side effects. And it is not very expensive.
I would recommend trying it for a few days and see that happens. I also recommend working with a lactation consultant or a La Leche leader to gt some guidance as you are building your supply.
For more information, I strongly recommend Kellymom.com. They have a wonderful section on herbs and breastfeeding, side effects, and what they do. I am not a paid spokesperson, I swear, lol!