Lemongrass grows very fast. To harvest them, grab a stalk and pull it from the ground. The white base is what is normally used in cooking.
Lemongrass is best grown in full sun and humid conditions. If the leaves seem to be drying out, aside from watering the plant, consider moving the pot into a shaded area and mist the plant to increase the relative humidity of the immediate surrounding.
Lemongrass can last up to 2 weeks in the refrigerator.