If it is boiled, last minute tops, just enough to sterilize it. My recommendation is to add it after the fermentation is completed, before bottling/kegging. If you do add them at the end of the boil, I'd probably multiply the quantity by 1.5x or even 2x.
With lemongrass, I get more aroma than I do flavor. the key is to crack the lemongrass before using it, therefor opening it up. Take each stalk of lemongrass (I used three for a 5.5 gallon batch, wouldn't hesitate to go four next time). Lay them across a cutting board and use the blunt end of a big knife to beat the stems, cracking them. You want a good thorough beating, I go probably at least a dozen whacks with the blunt end of a knife. Cut off the stalks. You just want the roots, it'll only be about 4-6 inches in length once cut. Dunk them and a muslin bag (and weights) in star san for several seconds to sterilize the stalks, and add to your beer. Go for 7 days to see if it's where you like it. If it's not enough, add another couple of stalks. It will pair very well with citrus IPAs and Saisons (especially if you use Sorachi Ace hops).